Cancer, old bean. Cancer.
The amount he smoked, it would have been a miracle for him to die of A.I.D.S.
You're right..Graham was openly gay..and I read some tabloid reports that it was AIDS..but this says cancer too.
here from http://www.lambda.net/~maximum/chapman.html
Chapman and Sherlock
Graham Chapman met his lover, David Sherlock, in 1966. Their enduring relationship is remembered fondly by some of the same people who were initially uncomfortable with the idea. In 1971, Chapman nursed John Tomiczek, a run away teenager from Liverpool, back to health. Sherlock and Chapman agreed to adopt Tomiczek on the condition that he finish school.
Their family endured the ups and downs of celebrity life, Chapman's battle with alcoholism, and painful cancer treatments. In the final days of his life, Chapman was confined to a wheelchair. Radiation treatments required lengthy commutes to London. Sherlock and Tomiczek were faithfully by his side through it all.
The Last Laugh
Chapman nearly drank himself to death before he finally gave up alcohol in the late 1970's. He later gave up smoking his trademark pipe. Despite these actions to preserve his health, he never displayed a fear of his own mortality.
"He always regarded death as highly overrated and
could never understand why anybody made such a fuss about it." ~ Michael Palin
In the film "The Meaning of Life", Chapman wrote and acted in a skit in which a man, condemned to die for the crime of making sexist jokes, is given the opportunity to choose his own form of execution. The character chooses to be chased off a cliff by a group of topless women wearing crash helmets.
Graham Chapman died of spinal cancer on October 4, 1989, the eve of the celebration of the 20th anniversary of Monty Python.
"I thought it was in terribly bad taste for him to die when he did." ~ Terry Jones
This is a good site too: http://www.geocities.com/fang_club/Chapman_biog.html