Billy Connolly realised he was suffering from Parkinson's disease after a fan noticed a problem with the way he walked.
The 71-year-old star was spotted by an Australian doctor, who was on tour with a group of dancers in America, and he insisted the comedian showed a "strange gait" which led to Billy being diagnosed with the degenerative nerve disorder.
Billy told a US radio station: "It was the strangest thing of all.
"I was walking through the lobby and every time I had gone through there was a crowd of boys and girls and a couple of adults.
"It turned out they were dancers from Australia.
"The guy who was in charge of them came over to me one day and said 'Billy, I'm a big fan, I'm from Tasmania'.
"He said, 'I'm a surgeon and I have been watching you walking, you have a strange gait'. That was the way he put it.
"He said 'You're showing distinct signs of early onset Parkinson's disease, see your doctor'.
"I think it was the way I held myself when I was walking.
"Then they did blood tests and various other little bits and pieces and told me I had it."
Billy - who is wed to psychologist Pamela Stephenson and has five children from his two marriages - also revealed he came off his medication because the side-effects were stronger than the effects of the illness, which can include shaking, rigidity, slowness of movement and difficulty with walking.
He added: "I don't see much shaking going on and I have always had a s**t memory anyway.
"I have forgotten things my entire career. I was taking drugs to control the Parkinson's but I'm not on them anymore.
"The doctor said the side effects were stronger than the effects. I never noticed any side effects but he told me they could include a deep interest in sex and gambling.
"In terms of treatment I think some of the stem cell stuff is very promising but they have a lot of problems legally getting it through."