The families of 12 probands with classic mitral valve prolapse were studied for evidence of mitral valve prolapse. Seventy parents, sibs, and progeny were included in the analysis. Forty-seven percent (16 of 34) of progeny were affected compared with 30 percent (3 of 10) of parents. Thirty-eight percent (10 of 26) of sibs were affected. A three-compartmental penetrance model was devised to account for the variation in expression with age. This includes a latent stage (time before onset of signs), an affected stage, and a stage in which the subjects are withdrawn (because of treatment, regression, or death). The implications of this model are discussed.