Former Lovin' Spoonful frontmanJohn Sebastian was given scripts for four shows, which was enough for him to identify with the show's underachieving students. After he wrote 'Welcome Back,' the producers named the show 'Welcome Back, Kotter' to fit the theme. DJs everywhere were swarmed with requests for the song, which soared to US No. 1 in 1976.

http://www.youtube.com/watch?v=9qy5LEeFHig&feature=player_embedded

Former Lovin' Spoonful frontmanJohn Sebastian was given scripts for four shows, which was enough for him to identify with the show's underachieving students. After he wrote 'Welcome Back,' the producers named the show 'Welcome Back, Kotter' to fit the theme. DJs everywhere were swarmed with requests for the song, which soared to US No. 1 in 1976.

http://www.youtube.com/watch?v=9qy5LEeFHig&feature=player_embedded

All very true, but I think the most known sitcom song in history, (not according to the charts) is The Monkees Theme.