Sir Billy Connolly "no longer recognises his close friends" as his Parkinson's disease worsens.
The 75-year-old comedian was first treated for the early symptoms of the degenerative disorder in mid-2013 and his close pal Sir Michael Parkinson says he was saddened during a recent meeting in New York when Billy did not know who he was.
Speaking on 'Saturday Morning with James Martin', Michael said: "The sadness of Billy now is that wonderful brain is dulled.
"I saw him recently - he's now living in America - and it was very sad, because I was presenting him with a prize at an award ceremony.
"We had an awkward dinner together, because I wasn't quite sure if he knew who I was or not. We were walking out after the presentation to go down and have our picture taken, and he turned to me and put his hand on my shoulders.
"He wasn't sure where [the dinner] was or what context at all."
Michael, 83, and Billy have been close friends for years, with Billy making a number of appearances on his talk show 'Parkinson', which ran from 1971 to 1982 and from 1998 to 2007, and he found the experience with his old pal deeply upsetting.
He added: "And to know someone as long as I knew and loved Billy... it was an awful thing to contemplate, that that had been taken from him in a sense.
'He was just a genius and the best thing that happened to me on the show."
Billy previously admitted he contemplated suicide on numerous occasions since being diagnosed with Parkinson's Disease.
Asked in 2016 if he has ever thought about ending his life, he said: "Yeah sometimes I give it a bit of thought when I'm in bed. I think, 'Well this is forever, this isn't going to get better, it's going to get worse.' But then I try and change my mind and I try and meditate and move away from it sideways.
"The guy who told me I had it said to me 'You know it's incurable?' I thought he could have said 'We have yet to find a cure' or something like that to put a bit of light at the end of the tunnel. I'm okay at the moment but it comes and goes. Sometimes I have trouble getting out of bed and I walk strangely. Turning over (in bed) is difficult. Turning from one side to the other can be quite complicated."