A survey was conducted on 12 050 Black schoolchildren, aged 2 to 18 years, in the South Western Townships of Johannesburg (Soweto), and the prevalence of non-ejection systolic clicks and late systolic murmurs was determined. One or both of these auscultatory findings were detected in 168 children,yielding a prevalence rate of 13-99 per 1000 in the school population. A female preponderance of 1b9:1 was present and there was a strong linear increase in prevalence with age, with a peak rate of 29-41 per 1000 in 17-year-old children. A non-ejection click was the only abnormal auscultatory finding in 123 children (73%) and a mitral systolic murmur in 8 (5%), whereas in 37 (22%) both these findings were present. Of the latter 37 children, the murmr was late systolic in 32; in 5 it was early systolic. Auscultation in different postures was important in the detection of both non-ejection clicks and mitral systolic murmurs. Experience in the detection of these auscultatoryfindings influenced thefrequency with which they were heard. Electrocardiographic abnormalities compatible with those previously described in the billowing mitral leaflet syndrome were present in 11 of 158 children. The aetiology of these auscultatoryfindings in this community remains unknown. In the same survey, a high prevalence rate of rheumatic heart disease was recorded and the epidemiology of the non-ejection clicks and these mitral systolic murmurs showed similarities to that of rheumatic heart disease. Though the specific billowing mitral leaflet syndrome almost certainly accountsfor some of these auscultatory findings, a significant proportion mey have early rheumatic heart disease. Further elucidation of this problem is necessary.