Idiopathic prolapse of the mitral valve is a common disorder, but many cases are clinically subtle. Thoracic skeletal abnormalities, reported recently to accompany the syndrome, may serve as an easily identifiable clinical indicator. The prevalence of these abnormalities was defined in 24 patients with proved prolapse of the mitral valve. The valvular syndrome was defined clinically, by echocardiography and, in seven cases, by left ventricular angiography. The skeletal deformities were defined clinically and radiographically. Pectus excavatum was present in 62 percent of the patients, "straight back" in 17 percent and severe scoliosis in 8 percent. Eighteen of the 24 patients (75 percent) had a definite thoracic skeletal deformity. The association of idiopathic prolapse of the mitral valve with these skeletal deformities may represent a forme fruste of Marfan's syndrome. Patients with "straight back" and pectus excavatum should be examined clinically and perhaps by echocardiography to exclude idiopathic prolapse of the mitral valve; when murmurs are present, a diagnosis of "pseudoheart disease" should not be made before mitral valve prolapse has been excluded.