The fourteen-year-old son of director Robert Altman, Mike Altman, wrote the lyrics to the theme song "Suicide is Painless." Because of its inclusion in the subsequent television series, he continued to get residuals throughout its run and syndication. His father was paid $75,000 for directing, but his son eventually made about $2 million in song royalties, with payments continuing, from first syndication through the present day, as M.A.S.H. (1972) continues in syndication around the world.
Tom Skerritt recalled that the dialogue was about 80% improvised. In order to create a different kind of atmosphere, Robert Altman cast some of the parts from improvisational clubs who had no previous movie or TV experience.
According to Johnny Mandel and Robert Altman, the film's famous theme song was intended to be the "stupidest song ever written." After attempting to write the lyrics himself, Altman said he found it too difficult to write "dumb enough," and instead gave to the task to his fourteen-year-old son. Mike Altman allegedly wrote the lyrics in five minutes, not even expecting to be paid, since he was the director's son. He ended up making millions in royalties off the song.
The first take of the shot where Hot Lips is revealed in the shower didn't work because Sally Kellerman anticipated the reveal, and was already lying on the floor when the tent flap went up. To distract her, Robert Altman and Gary Burghoff entered the shower tent and dropped their trousers while the shot was rolling outside. While Kellerman was staring at them, the tent flap was raised, resulting in her genuine surprise and shock when she realized what had happened. In the Special Edition double disc DVD, they say that Radar (portrayed by Gary Burghoff) had been standing naked beside the camera, and that's the reason why Sally Kellerman looks so surprised when the flap was raised.
Robert Altman said that during filming, Elliott Gould and Donald Sutherland together went to the studio and complained that Altman was filming too much of the secondary characters. They requested that he be removed from the film, but the studio refused. After the film was completed and received its accolades, only Elliott Gould confessed the matter to Altman. As a result, he received parts in other Altman pictures, whereas Robert Altman never cast Donald Sutherland again.