3 Answers

He died of Prostate Cancer and wrote this in his final letter: 

 "My friends, love is better than anger. Hope is better than fear. Optimism is better than despair. So let us be loving, hopeful and optimistic. And we'll change the world."

Excerpts were taken from the following:

1.  Read more: http://www.care2.com/causes/jack-layton-dies-of-cancer.html#ixzz1Wqz7sO6A  "It was already well known that Layton was battling prostate cancer, however, that cancer appeared to be under control." 

2.  A statement that he died of prostate cancer was also published in  http://www.bclocalnews.com/opinion/128684898.html

 3.  It is also stated in www.en.wikipedia.com that he died of the same illness his father died of (Robert Layton, Jacks Father died May 9, 2002(2002-05-09) of prostate cancer.) 

Let me find my source and I'll get back to you.  yvonne57