"What a Fool Believes" was one of the few non-disco No. 1 hits on the Billboard Hot 100 during 1979. The song lyrics tell a story of a man who is reunited with an old love interest and attempts to rekindle a romantic relationship with her before discovering that one never really existed.
Michael McDonald reputedly fired the whole band along with the stage manager after a 1979 Chicago show in which a backup singer hit a flat note on the second chorus for this hit. In an interview later in the year with Rolling Stone, McDonald revealed he "couldn't abide such mistakes. I was running a band, not a circus."
Michael Jackson once claimed he contributed at least one backing track to the original Doobie Brothers recording, but was not credited for having done so.  This was later denied by the band.
Warner Brothers also released a 12" single disco version by The Doobie Brothers (backed with "Don't Stop to Watch the Wheels"). Mixed by disco producer Jim Burgess, at 5:31 the song is considerably longer than the 3:41 versions on the 7" single and the Minute by Minute LP. The 12" version also has a more pronounced bass-driven drumbeat.
Live versions of the song often feature a guitar bridge after the second verse.
A fictional account of the creation of this song was featured in the first episode (June 2005) of Yacht Rock, an Internet series featuring characters of Michael McDonald and the Doobies. In the parody, Kenny Loggins inspires McDonald to write the song after Doobie guitarist Jeff "Skunk" Baxter threatens to kick McDonald out of the band unless he writes a hit song for them.
A cover of Petula Clark's "Downtown" on the Canadian sketch comedy series SCTV includes a variation of the beginning of "What a Fool Believes". A Michael McDonald impersonator sings backup in the song.
Another Michael McDonald lookalike sings the song's melody with different words in the Late Night with Conan O'Brien sketch "The Camp Michael McDonald Channel". The McDonald impersonator also sings different words to the tune of "Takin' It to the Streets" and McDonald's solo hit "Sweet Freedom".
MF Doom sampled the song in "Mandrake", an instrumental beat on his Metalfingers Presents: Special Herbs, The Box Set collection.
The song is also featured in a Toyota Japanese TV commercial "Corolla Hybrid Jeans" where the mayor of Toyotown (Takuya Kimura) and two Toyota Corolla Fielder Hybrids in his garage are featured, with the commercial theme, "Love and Jeans".
The song was featured in Grand Theft Auto V on the in-game radio station Los Santos Rock Radio, where the song's co-writer, Kenny Loggins, is the station's DJ. It is more prominently featured in the mission 'Crystal Maze' where character Trevor Phillips, after growing furious over a fallen business deal, enters his truck to hear the song playing. He screams "Ah, this music's all fucking wrong!", shortly before changing the channel to punk station Channel X.
The song was featured in the Stone Quackers episode "Stone Quackers II", in which one character is tied to a chair and forced to listen to the song so that he will be healed of his obsession with a girl who has broken up with him.
^There is a video of this performance on YouTube titled: "Kenny Loggins What A Fool Believes Live 1992".
^What A Fool Believes (12"), discomusic.com. They comment: Disco from an unlikely artist ... "What A Fool Believes" was remixed by the late Jim Burgess to enhance its dance floor appeal. Another good Doobie Brothers 12 inch release was "Real Love".
^This 12" single was recorded at 33⅓ rpm, which enables better bass response; see 12-inch single for more information.
^ abAretha Franklin has two versions of this song, with the 1999 version being an edited version of the 1980 one.
^George Michael at musiclovers.de. What a Fool Believes appears as track 14 on this bootleg recording, and is designated as one of George Michael's "rare recording"s. Most recently retrieved: 2007-09-14.
^The Wades at Soul and funk music.com. Most recently retrieved: 2007-09-14.