"(I Don't Know Why) But I Do" is an R&B song written by Paul Gayten and Bobby Charles (as Robert Guidry), and performed by Clarence "Frogman" Henry. It was Henry's biggest U.S. hit, reaching #4 in early 1961. The B-side on the single release was "Just My Baby and Me".

The U.S. release on the Argo record label was titled "I Don't Know Why". The U.K. release on the Pye label and the Australian release on the Coronet label were both titled "But I Do". The U.K. version spent 19 weeks in the charts and peaked at #3 in the first week of May 1961.

The song was made popular again after appearances in the 1994 film Forrest Gump, and the 1999 film Mickey Blue Eyes.

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