<P> "I Know Why (And So Do You)" is a 1941 song by Glenn Miller and His Orchestra . The song appeared in the 20th Century Fox movie Sun Valley Serenade . The song was also released as an RCA Bluebird 78 single . </P> <P> The song was written by Mack Gordon, lyrics, and Harry Warren, music . The song is lip - synched by Lynn Bari in the movie Sun Valley Serenade . Pat Friday sang the vocals with John Payne and The Modernaires . The single, RCA Bluebird B - 11230 - A, reached no . 18 on the Billboard pop singles chart in a one - week chart run . The single release featured vocals by Paula Kelly and The Four Modernaires . The B side of the single was "Chattanooga Choo Choo", which de facto, was treated as the A side . </P> <P> The Manhattan Transfer, Mel Torme, Richard Himber And His Orchestra with Johnny Johnston and the Joseph Lilley Ensemble on vocals, Ray Anthony, Anne Shelton with Ambrose & his Orchestra, June Christy, Frankie Sardo on 20th Fox Records, Lionel Hampton, Jean Peters, Joan Regan, Michael Feinstein and George Shearing, Babik Reinhardt, Dick Smothers, the Red Garland Trio, Orchestra Coco, the Michael Rose Orchestra, Teddy Petersen, Falconaires, US Air Force Band of the Rockies, Max Greger, Syd Lawrence, the Dino Olivieri Orchestra with refrain by Bruno Pallesi, David Pell, Diana Panton with Red Schwager on the album Red, and Robert Clary have also recorded the song . </P>

Who wrote i know why and so do you