Also known as Suicide is Painless, lyrics by Robert Altman's 14-year-old (at the time) son. Through an interesting circumstance related to publishing, Mike Altman got a royalty from every rerun of MASH, despite the fact that the lyrics aren't used on the show. He had a super-bitchin' house up on Camano Island in Washington. I dated a girl who went to high school with his son, whom I also later worked with on a show, as well as Robert Altman's other son, Bobby, cinematographer responsible for "the Player pull:" Both Bobby and Jade were f'n awesome. I was sitting down to lunch once and Bobby said "I'm really boning you in all these shots." (Bobby likes to use lots of cameras, which makes it really hard to get a boom in, and the show was... challenging). "It's all good, Bobby, it's part of the gig." "No, but you're totally handling it. My dad would have loved working with you." Probably the highest praise I'll ever receive for sound, and I'm A-OK with that. I was walking on air for weeks.
Apparently Robert decided it had to be called suicide is painless and that it needed to be "the stupidest song ever written". He struggled with it himself for a while and then had Mike write it, who apparently managed to throw it together in about five minutes. Here's an interview that details the subject. Though I suppose you personally have probably heard the story first hand!
It's almost all on Netflix, just added this year in 2 big chunks. Maddeningly, the final 2 hour episode is missing out of the bunch. It's well worth the watch on Netflix, as you can binge big chunks. It really brings out how quickly the show changed and morphed, from the big changes after season 3, to the more mature subjects as the show went on.
It's a funny quirk of life that you posted this; when I would get severely depressed I would listen to Johhny Cash - Hurt, this song, and a short environmental song from Star Wars Galaxies that would play when it rained. It was silly but it became my ritual, Cash let me know I wasn't alone, MASH made me think of what would happen and gloom_b let me know that you have to accept the past but the future brings change. I've long since grown out of this ritual, but at a time I felt pathetic and uncomfortable with myself it helped me to step back and cope.
What a great show. I recommend watching the whole thing. Fun fact: MASH was not aired in chronological order! If you watch the episodes in the order they aired (which is the standard order), you'll notice weird little things like the season changing from episode to episode. There's also one season in which they invent a vein clamp, so in addition to the episode where they actually come up with it there's an episode where they show them to another military hospital as well as at least one other episode where they reference the clamp. Well as you might have guessed by now, the network aired the other episodes involving the clamp before they actually invented it. The way episodic TV was at the time everything was usually back to normal by the end of the show, so this was apparently a pretty standard practice.