This information is from The New York Times via:
http://www.nytimes.com/2009/12/21/movies/21obannon.html
Dan O’Bannon, whose screenplays for “Alien,” “Total Recall,” “The Return of the Living Dead” and other films made him a cult hero among science fiction aficionados, died on Thursday in Santa Monica, Calif. He was 63.
The Writers Guild of America confirmed his death. The cause was Crohn’s disease, a chronic gastrointestinal disorder that Mr. O’Bannon endured for 30 years, his wife, Diane, told The Los Angeles Times.
Mr. O’Bannon had an early start as a screenwriter when he and the director John Carpenter, students at the time at the University of Southern California film school, wrote the low-budget film “Dark Star,” which was released as a feature in 1974.
After working as a computer animator for the director George Lucas on “Star Wars” and trying, unsuccessfully, to develop a film based on the Frank Herbert novel “Dune,” Mr. O’Bannon created the story of “Alien” with the screenwriter Ronald Shusett and wrote the screenplay on his own.
The film, directed by Ridley Scott and starring Sigourney Weaver, is about a spaceship with a vicious monster loose onboard. (The creature begins as a parasite that explodes from a crew member’s chest.) It became a box office hit, a classic of science fiction and horror, and the progenitor of a lucrative Hollywood franchise, with its several sequels.
“I love gore films and I grew up with ’50s monster movies,” Mr. O’Bannon told the journal Cinefantastique in 1979, speaking of the film’s origins. “The idea for the monster in ‘Alien’ originally came from a stomach ache I had.”
In 1985 Mr. O’Bannon wrote and directed “The Return of the Living Dead,” part homage to the George Romero zombie film “Night of the Living dead” and part genre spoof. In 1990 he teamed with Mr. Shusett again, among others, to write “Total Recall,” a violent, futuristic tale set partly on Earth and partly on Mars and based on a short story by Philip K. Dick. It starred Arnold Schwarzenegger and Sharon Stone.
Daniel Thomas O’Bannon was born in St. Louis on Sept. 30, 1946. He attended Washington University in St. Louis and MacMurray College in Abilene, Tex., before earning an M.F.A. from U.S.C.
His other screenwriting credits include “Blue Thunder” (1983); “Lifeforce” (1985) and “Invaders From Mars” (1986), both directed by Tobe Hooper (who directed “The Texas Chain Saw Massacre” and “Poltergeist”); “Screamers” (1995); and “Bleeders” (1997).
He also directed “The Resurrected” (1992), based on a story by H. P. Lovecraft, whom Mr. O’Bannon called “the greatest horror writer who ever lived.”
Besides his wife, he is survived by a son, Adam.
Condolences go out to the family - Dan will live on in the hearts and memories of many people, as well as an incredible legacy.