A manifesting carrier of Duchenne muscular dystrophy with severe myocardial symptoms. A 42-year-old so-called manifesting carrier of Duchenne muscular dystrophy (DMD), whose first complaints were severe myocardial symptoms, is described. Immunohistochemical study using anti-dystrophin anti-serum and analysis of cloned segments of X chromosome DNA were performed. Her two sons and one of her brothers appear to have had the same disease. She was admitted to the hospital complaining of dyspnoea, back pain and palpitations and was first diagnosed as having myocardial infarction. However, this diagnosis was excluded. The echocardiogram showed diffuse abnormalities of myocardial function. Serum enzymes were increased. Minimal weakness and decreased deep tendon reflexes were detected in her left lower extremity. Muscle biopsy revealed a small number of necrotic fibres. Immunohistochemical study using anti-dystrophin antiserum showed a mosaic pattern of the surface membrane. Analysis of cloned segments of X chromosome DNA from the patient and her son showed the XmnI(Asp) alleles of pERT 87-15 and the TaqI alleles of pERT 87-8 in both patients. 