Well, the reign of heavyweight champ Vitali Klitschko is over before it really got started. The giant Ukrainian was supposed to fight former titlist Hasim Rahman last week, but a bum knee – the latest in a long line of Klitschko injuries – forced the fight’s cancellation.

And, ultimately, the termination of Klitschko’s career.

The end came suddenly. But that’s certainly not the first time the career of an active champion in his prime came to a close without warning. Here are a few other champions who retired on a whim.

It’s no surprise that most of them came back.

* Sugar Ray Leonard: The great multi-division champion was in the prime of a glorious career when he shockingly retired in November 1982 – a mere 14 months after his defining knockout of Thomas Hearns and just nine months removed from his previous fight, a third-round knockout of no-hoper Bruce Finch. The culprit: a detached retina, possibly suffered in the Hearns fight. Leonard’s retirement, announced at a pompous Baltimore gala, seemed unfair at the time, since the boxing world appeared cheated of an important part of Leonard’s career. But, predictably, Sugar Ray came back … again and again and again and again. He returned in 1984, ’87, ’88, ’91 and ’97. Some of the comebacks were successful (W 12 Marvin Hagler), some of them were not (KO by 5 Hector Camacho). Leonard became known for calculated retirements and returns which, for many fight fans, resulted in a serious dislike for the all-time great fighter.

* Felix Trinidad: Things seemed to be rolling right along when “Tito” surprised everybody and retired in 2002. Sure, it was just a few months removed from his first career loss to Bernard Hopkins in 2001. But Trinidad had impressively stopped Hassine Cherifi in four rounds in his follow-up effort, and he and Hopkins were roaring toward a rematch. But negotiations fell through, convincing Trinidad to step away. The official reason: He wanted to spend more time with his family. Too bad, because he wasted two years of his prime with the pointless hiatus. He returned in 2004, stopped Ricardo Mayorga in an exciting fight, and was again talking Hopkins rematch. Then he opted to fight the impossible Winky Wright – definitely not a good career move – and was shut out over 12 rounds. End of career #2. We expect career #3 to kick off sometime in 2006 or ’07.

* Leo Randolph: A 1976 Olympian on perhaps the greatest amateur boxing team in history, Randolph turned pro in 1978 and reeled off eight wins before being upset by David Capo in 1979. But that didn’t deter the Tacoma, Washington, native, who posted another eight victories and earned a shot at WBA super bantamweight champ Ricardo Cardona in 1980. Randolph – known as “The Lion” – stopped Cardona in the 15th round in Seattle, and a star seemed to be born. Not so fast. Randolph was upset in his very first title defense, by underdog Sergio Palma, and lost via 5th-round knockout just three months after winning the crown. Then, shockingly, Randolph announced his retirement, and that was it. He never returned, he never contemplated a return, and his boxing career was history. He retired with a record of 17-2 (9 knockouts).

* Marvin Hagler: The former undisputed middleweight champion of the world never officially announced his retirement. He lost via a shocker to Leonard in April 1987, and simply never fought again. What made Marvelous Marvin’s non-return so unlikely is that the Brockton, Mass., fighter stood to make millions in rematches with Leonard and Thomas Hearns. But he resisted, took off to Italy to become a movie star, and never put on another pair of gloves. Today, he seems content and happy, hardly showing the bitterness that marked the months immediately following the Leonard loss. Which must mean that walking away was the right move for one of the greatest 160-pounder of all-time.

So there are four fighters who retired unexpectedly. Two of them returned, and two of them did not. When his injuries heal, Klitschko will seriously consider a comeback. And it will be hard to resist.

Probably impossible to resist.