Robert Altman originally wanted Elliot Gould to play Duke Forrest; it was only at Gould's request that he played Trapper John.

Approximately half way through the movie, there's a shot of the moon with an announcement made over the system, as this was filmed on the exact same night that the first humans made their walk on the moon. From the audio commentary.

Gary Burghoff as Radar was the only actor who appeared in both the movie and the TV series. [If you want to count just one show as an appearance, then G. Wood, the actor who played General Charlie Hammond from the 325th Evac in the movie, the one with the football team, was also in the pilot episode of the TV show where Hawkeye announces that Father Mulcahy is the winner of the trip to Tokyo with 'the lovely Lt. Dish'. I'm not sure that he appeared in any more episodes, but he did appear in that one. Also, Gary Burghoff was the only actor to play the same role in the movie as well as the series, but there were several other actors appearing in both.]

Director Robert Altman's 14-year-old son, Mike, wrote the lyrics to the MASH theme "Suicide Is Painless," and in the end received more money in royalties than his father did for directing it. http://www.absoluteastronomy.com/encyclopedia/m/mi/mike_altman.htm

All of the characters, based on the characters from Richard Hooker's novel, are composites of people Hooker knew, met casually, worked with, or heard about.

Director Robert Altman cast so many unknowns, that after the few known actors (Donald Sutherland, Elliot Gould, etc.), the opening credits are entirely "Introducing..."

During filming, Donald Sutherland and Elliott Gould spent a third of their time trying to get Robert Altman fired.

When the dentist (painless) is going "to kill" himself, the officers are giving him a last dinner. And they are sitting behind the table in exactly the same positions as in Leonardo da Vinci's last supper.

In one of the opening scenes, one of the corpsmen is running from a helicopter while carrying one end of a stretcher and he accidentally slips and falls. The director thought it was an appropriate blunder and decided to leave it in.