We report a case of symptomatic carrier of Duchenne muscular dystrophy (DMD) in a 14-year-old girl with no prior family history of DMD. She presented with chest pain, enlarged calf muscles, an elevated serum creatine kinase (CK), and decreased left ventricular function. Histological examination of skeletal muscle revealed myopathic changes and immunostaining with anti-dystrophin antiserum demonstrated a mosaic pattern which are compatible with the observations in carriers of DMD. Southern blots using the dystrophin cDNA revealed no evidence of a deletion within the DMD gene in the patient or in her mother. We found this observation interesting for two reasons: 1. Cardiomyopathy is rare in female DMD patients. 2. Immunostaining of a muscle biopsy with anti-dystrophin serum proved to be valuable in the diagnosed for symptomatic carriers of the dystrophin gene mutation.