It was Henry's biggest U.S.hit, reaching No. 4 in early 1961. The B-side on the single release was "Just My Baby and Me".
On its initial release in December 1960, the U.S. release on the Argo record label was titled "I Don't Know Why". However, about ten weeks later, Argo announced that due to confusion arising from the song being mistaken for the 1931 song called "I Don't Know Why (I Just Do)", they changed the name of this song to "But I Do". The UK release on the Pye label and the Australian release on the Coronet label were both titled "But I Do". The UK version spent 19 weeks in the charts and peaked at No. 3 in the first week of May 1961.
Bobby Vinton covered "But I Do" in 1972. His version reached No. 82 on the U.S. Billboard Hot 100 and No. 71 on the Cash Box chart in early 1973. It also reached #27 on the Adult Contemporary chart.
The song also charted in Canada on both the Pop chart (#72) and Adult Contemporary (#23) chart, where it made its best showing.