Echocardiography, including Doppler analysis, was performed to assess the prevalence of cardiac abnormalities in 163 patients with autosomal dominant polycystic kidney disease, 130 unaffected family members, and 100 control subjects. In these three groups, the prevalence of mitral-valve prolapse was 26, 14, and 2 percent, respectively (P less than 0.0005). A higher prevalence of mitral incompetence (31, 14, and 9 percent, respectively; P less than 0.005), aortic incompetence (8, 3, and 1 percent, respectively; P less than 0.05), tricuspid incompetence (15, 7, and 4 percent, respectively; P less than 0.02), and tricuspid-valve prolapse (6, 2, and 0 percent, respectively; P less than 0.02) was also found in the patients with polycystic kidney disease. These findings reflect the systemic nature of polycystic kidney disease and support the hypothesis that the disorder involves a defect in the extracellular matrix and the cardiac abnormalities are an expression of that defect.