We searched for mitral-valve prolapse by two-dimensional echocardiography in 15 patients with von Willebrand syndromes to test the hypothesis that this bleeding disorder is actually a mesenchymal dysplasia that resembles the heritable disorders of connective tissue. This valvular abnormality was found in nine (60 per cent) of these patients, as compared with four (13.3 per cent) of 30 sex-matched and age-matched healthy controls. This difference was statistically significant (P less than 0.01). The association between these two disorders encourages a search for mitral-valve prolapse in persons with a von Willebrand syndrome. The complex of a von Willebrand syndrome and mitral-valve prolapse may be an example of a newly recognized category of related coagulation and cardiovascular disorders.