Billy Connolly, who is being treated for the "initial symptoms" of Parkinson's Disease, admitted earlier this year that he had started to forget lines during performances.
Speaking about it, he said: "This is f*****g terrifying. I feel like I'm going out of my mind."
There is no cure for the disease but symptoms can be controlled using a combination of drugs, therapies and occasionally surgery.
Actor Bob Hoskins announced his retirement last year after being diagnosed with the disease.
Billy Connolly was born in Glasgow in 1942 and after leaving school he worked in the shipyards and later joined the Territorial Army.
He learned how to play the banjo and joined a band called the Humblebums, which featured Gerry Rafferty, who later recorded the song Baker Street.
Connolly told jokes between songs and later became a full-time comedian with one of his major breakthroughs coming after an appearance on chat show Parkinson in the early 1970s.
As well as comedy, he has also turned his hand to acting with roles in The Last Samurai and the forthcoming film The Hobbit: There and Back Again.
Connolly is also known for his charity work with frequent appearances on Comic Relief - including one year when he streaked through central London.
The comedian was made a CBE in 2003 and was awarded the freedom of his home city in 2010.
Comic and actor Billy Connolly has undergone surgery for prostate cancer and is being treated for the "initial symptoms" of Parkinson's Disease, according to a spokeswoman for the star.
Scottish actor and comedian Billy Connolly has vowed to stay out of political discussions after a backlash to his opinions on Scottish independence.
The 69-year-old said: "Whenever I open my mouth I get it slapped shut. I'm told to mind my own business, to learn what I'm talking about".
The star went on to express his dislike of celebrities influencing the public on who to vote for.
Connolly has openly said in the past that he does not approve of Scottish independence.
he was speaking at the premiere of his latest film Quartet, a British comedy directed by Dustin Hoffman.