A total of 1934 subjects aged 11 to 70 were screened phonocardiographically for mitral valve prolapse. Systolic click, systolic murmur or a combination of the two were found in 2.64 +/- 0.36% of the subjects (a 95% confidence margin). The incidence of mitral valve prolapse was not significantly dependent on sex and age. In subjects with mitral valve prolapse, the phono- and echocardiographic findings did not always correlate.