To determine the prevalence of mitral valve prolapse in ballet dancers by echocardiography and to establish which anthropometric measurements best predict the presence of mitral valve prolapse, we compared 44 professional dancers (22 men and 22 women) with 52 controls (24 men and 28 women). Forty-eight percent (21 of 44) of dancers had echocardiographic mitral valve prolapse compared with 10% (5 of 52) of controls (p less than 0.01). The dancers weighed less than the controls and had significantly smaller bony diameters and body circumferences. However, only ponderal index was predictive of mitral valve prolapse in both dancers and controls. Thus, echocardiographic mitral valve prolapse is associated with low body weight relative to height and neither to ballet dancing nor to a distinct body habitus. Echocardiographic mitral valve prolapse may represent a normal variant in the majority of asymptomatic, thin subjects without auscultatory findings.