One (Harry Nilsson song)

"One" is a song by American singer-songwriter Harry Nilsson from his third studio album Aerial Ballet (1968). It is known for its opening line "One is the loneliest number that you'll ever do". Nilsson wrote the song after calling someone and getting a busy signal. He stayed on the line listening to the "beep, beep, beep, beep..." tone, writing the song. The busy signal became the opening notes.

A better-known cover version, recorded by Three Dog Night, reached number five on the U.S. Billboard Hot 100 in 1969 and number four in Canada. In 1969, the song was also recorded by Australian pop singer Johnny Farnham, reaching number four on the Go-Set National Top 40 Chart.