Former heavyweight champ Muhammad Ali, his body gripped by Parkinson's disease but his eyes flashing a fighting fire, has said boxing was a tougher challenge than the illness he has battled for 16 years.

"I'm still the greatest," the 55year-old fighter said in a voice little more than a whisper during a news conference in Chicago yesterday where he clowned with his wife, joked and made a red silk handkerchief disappear by sleight of hand.

The rare public discussion of Ali's health came at a briefing called by a drug company, Pharmacia & Upjohn, to talk about medical advances in treating Parkinson's disease.

Ali has been taking one of the company's drugs, Mirapex, which was approved for the US market in July. But his wife, Lonnie, told reporters the most dramatic improvement in his condition recently came from a different, 50-year-old drug that has brought back his ability to smile.

"There came a time when Muhammad was not smiling," she said, but a drug called Artane relaxed facial muscles that had been frozen by the disease.

"There's a million and a half people with Parkinson's who've never boxed," the champ added.

Asked which was harder, fighting Parkinson's disease or opponents in the ring, Ali answered "boxing's tougher".