The opening segment of a forthcoming autobiography by Sugar Ray Leonard runs counter to the cunning style he used in winning boxing championships in five weight divisions more than a quarter-century ago. It is more like hearing the bell, rushing to the center of the ring and being hit with a straight right hand.

Most fans of Leonard remember him for his sweet smile and lightning-fast hands, as a transcendent and breakout celebrity in a brutal profession. But by Page 36 of “The Big Fight: My Life In and Out of the Ring,” to be published next month by Viking, Leonard has mentioned his cocaine use, growing up in a home with alcohol abuse and domestic violence, luckily surviving a car wreck with his mother at the wheel, almost drowning in a creek as a child who was unable to swim, and fathering a son at 17.

Two pages later, Leonard delivers the book’s bombshell while indirectly addressing a growing concern in the sports industry at large. He reveals publicly for the first time that he was sexually abused as a young fighter by an unnamed “prominent Olympic boxing coach.”

Leonard writes that when the coach accompanied him as a 15-year-old and another young fighter to a boxing event in Utica, N.Y., in 1971, he had the teenagers take a bath in a tub of hot water and Epsom salts while he sat on the other side of the bathroom. They suspected “something a bit inappropriate” was occurring but did not want to question a strong male authority figure.

Several years later, Leonard describes sitting in a car in a deserted parking lot across from a recreation center, listening intently as the same coach, said to be in his late 40s, explained how much a gold medal at the 1976 Olympics would mean to his future.

Leonard was flattered, filled with hope, as any young athlete would be. But he writes: “Before I knew it, he had unzipped my pants and put his hand, then mouth, on an area that has haunted me for life. I didn’t scream. I didn’t look at him. I just opened the door and ran.”

He adds that when he first decided to discuss the incident in the book, which is written with Michael Arkush, he offered a version in which the abuser stopped before there was actual contact.

“That was painful enough,” Leonard writes. “But last year, after watching the actor Todd Bridges bare his soul on Oprah’s show about how he was sexually abused as a kid, I realized I would never be free unless I revealed the whole truth, no matter how much it hurt.”

Through his publisher, Leonard, who turned 55 on Tuesday, declined to comment for this article, saying that he would begin doing publicity for the book in June. But several people who were close to him when he was routinely banking multimillion-dollar purses for title bouts with Roberto Duran, Thomas Hearns and Marvin Hagler were taken aback when told of what he has revealed in the book.

“This is the first time I’ve ever heard that, and I’ve known Ray since he was just a kid,” Dave Jacobs, who was Leonard’s first trainer as an amateur and later served as assistant trainer for many of his professional fights, said in a telephone interview. “He never talked about that to me and no one in the group ever mentioned it, so I assume he never talked about it to them, either.

“But if that incident did happen, I feel sorry for him in that part of his life and for having to carry that around with him.”

Angelo Dundee, who achieved fame as Muhammad Ali’s trainer and later became the head man in Leonard’s corner, said he knew very little about his fighter’s personal lives and preferred it that way.

“Ray never mentioned anything, but I never mingled with anything to do with a fighter except fighting,” Dundee said from his Florida home. “You never wanted personal stuff getting in the way when you sent a kid into the ring. And as far as I could see, Ray was as mentally tough as they came.”

As the president of HBO Sports, Ross Greenburg was involved in the television negotiations for many of Leonard’s fights and employed him as an analyst during Leonard’s retirements, which turned into sabbaticals. Greenburg said he was aware only of Leonard’s drug and alcohol use, which was made public in 1991 when Leonard responded to a report in The Los Angeles Times culled from records of his divorce from his first wife, Juanita.

“During his semiretirements, Ray would fall apart, slip into the drugs and drinking,” Greenburg said. “He really needed the prefight routine, all the stuff in the gym. He was working for us, but he was way too young to be in semiretirement.”

In addition to his boxing triumphs, Leonard’s book details his travails with drugs, alcohol, infidelity and domestic problems. In certain respects, he describes the instability as a reprise of his life growing up as the fifth of seven children born to Cicero and Getha Leonard.

According to Leonard, his parents’ relationship was loving and lasting but filled with turbulence unfit for a child’s eyes. He recounts an instance in which his mother stabbed his father in the back with a switchblade he had shown her how to use, sending him bloodied into the street in search of someone to remove it.

Leonard admits that his relationship with Juanita and their sons, Ray Jr. and Jarrel, suffered similarly. But his story ends happily with him remarrying, starting a second family and finally admitting he was an alcoholic. Greenburg speculated that it was counseling that helped him finally come to grips with the sexual-abuse episode of his youth.

“Having to hide a situation like that made it worse, I would think,” he said. “You have these dirty little secrets, and you feel as a man, and one in a tough-guy world like boxing, that you can’t share it with anyone. I would think that would probably affect every aspect of his life.”

Almost four decades after Leonard’s professed incident, the United States Olympic Committee is grappling more openly with the subject of coaching predators. Last year, after an ABC News investigation found that 36 swim coaches had been barred by USA Swimming after allegations of sexual misconduct, the U.S.O.C. responded by forming a task force to study the problem.

When Nina Kemppel, a four-time Olympian in cross-country skiing, was appointed to head the task force, she received numerous calls from athletes, parents and coaches who had experienced or had knowledge of sexual abuse. Last September, the U.S.O.C. agreed to implement the task force’s recommendations to centralize and standardize ways for sports to run background checks on the coaches they certify, and to take a leadership role in emphasizing safe training environments.

In a telephone interview, Kemppel said sports often created an environment where “there is a relationship of intimacy and closeness and where coaches and athletes spend an unusual amount of time together.”

That seems to be what Leonard describes, a belief that the unnamed coach held his future in his hands. Later in the book, he speaks of haunting flashbacks and says that the experience, and a volatile home life as a child, contributed to his misadventures as an adult.

Comparing Leonard with fighters not as fortunate in their boxing afterlives, Jacobs and Dundee still view him as a blessed man, floating above the fray. Both fell out with the Leonard camp along the way but maintain affection for him, they said, happy to see him in recent years on a boxing reality show and on “Dancing With the Stars.”

“I was never worried about Ray going over the deep end,” Jacobs said. “Since Ray was 14, I always saw him as someone who would get the best out of life.”

According to Greenburg, rare is the champion who was safeguarded as well as Leonard was by his friend and lawyer Mike Trainer, who guided his business interests from the beginning.

“Whatever is in the book, he is no sad boxing story, and no one can ever turn him into that,” Greenburg said. “He’s got his money. I lived it, negotiated it, and I can tell you that there’s no manager who ever did a greater job for a fighter than Mike Trainer.”

Greenburg speculated that Leonard’s inclusion of the sexual abuse in the book was a sign of strength and growth after all he has been through.

“Most people wouldn’t even write a book like this, much less admit to something as personal as sexual abuse,” he said. “But knowing Ray, I’d bet he’s thinking that if one person reads this and says, ‘Oh, that happened to me, too, and I need to get help,’ then he’ll think it was worth it.”