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Myocardial function was evaluated prospectively by noninvasive methods in 20 boys with clinical, biochemical, muscle biopsy, and electromyographic evidence of Duchenne's progressive muscular dystrophy. Auscultatory evidence of a nonejection systolic click suggested mitral valve prolapse (MVP) syndrome in seven patients. Phonocardiography disclosed that the click was mid-systolic in four patients and early in three. Echocardiographic features consistent with this diagnosis were identified in all seven patients and in an additional four. One of these had an apical pansystolic murmur, suggestive of mitral regurgitation, whereas in the other three, prolapse of the mitral valve was "silent". Echocardiographic findings included an abrupt midsystolic, posterior motion (greater than 3 mm beyond the CD line) in five patients, multiple sequence echoes in six, and posterior coaptation of the mitral valve near the left atrial wall in six. The features most characteristic of MVP syndrome was a smooth, pansystolic, anteriorly concave (hammock-like) posterior motion deviating more than 3 mm beyond the CD line. Among the remaining nine patients who did not have echocardiographic evidence of prolapsing mitral valve, none had an early, middle or late nonejection systolic click or a heart murmur, although four patients in this group had moderate to severe scoliosis. These observations document of occurrence of MVP syndrome in children with Duchenne's muscular dystrophy and indicate that its prevalence is high. We speculate that prolapse of the mitral valve in these patients is an expression of the underlying cardiomyopathy characteristic of Duchenne's muscular dystrophy rather than an isolated, dystrophic involvement of the mitral valve leaflets.