John Cazale died of bone cancer in 1978 aged 42. Tragic loss as he was a fine actor.

Taken from his Wiki page:

During his six-year film career, he appeared in five films, each of which was nominated for the Academy Award for Best Picture: The Godfather, The Conversation, The Godfather Part II, Dog Day Afternoon and The Deer Hunter. He is the only actor to have this multi-film distinction