Cardiac dysfunction and its correlation with skeletal muscle dysfunction were examined in 16 definite female gene carriers of Duchenne muscular dystrophy (DMD). Five out of 16 carriers (31.3%) had cardiac symptoms and 8 carriers (50.0%) showed an increased cardio-thoracic ratio on chest X-ray. Electrocardiographic abnormalities including a high R:S ratio (> or = 1.0) in the V1 lead, deep Q wave (> 3 mm) in the I, II, aVL, V5, and V6 leads, complete right bundle branch block and premature ventricular beats, were observed in 9 carriers (56.3%). On echocardiographic examination, an increase in the end-diastolic dimension of the left ventricle and a decrease in the ejection fraction suggestive of dilated cardiomyopathy were found in 12 carriers (75.0%). Tl-201 myocardial SPECT scan was performed in 2 symptomatic carriers and showed an area of hypoperfusion in the inferio-posterior wall. These findings were similar to previously reported findings in DMD patients. A biopsy of the myocardium was obtained in one carrier with her informed consent for the biopsy. Immunohistochemical staining demonstrated that 75.4% of the myocardial fibers were negative for dystrophin, suggesting that her cardiac dysfunction is caused by the abnormal expression of dystrophin in the cardiac muscle. On examination of the skeletal muscle function, none of the carriers had clinical evidence of muscle weakness or atrophy. However serum creatine kinase activity was elevated in 14 of 16 carriers (87.5%). Computed tomography (CT) of the lower limb muscles demonstrated widened spaces among muscles and moss-eaten appearance of low density areas within muscles and CT value was decreased, suggesting the subclinical involvement of the skeletal muscle. In the carriers without cardiac symptoms, there was a negative correlation (p < 0.05) between the end-diastolic dimension of the left ventricle and the CT value of the biceps femoris muscle (a muscle with the lowest CT value among the lower limb muscles). This indicates that there was an apparent correlation between the cardiac and skeletal muscle dysfunction. These findings suggest a high frequency of clinical and subclinical involvement of the cardiac and skeletal muscles in DMD carriers. To protect them from cardiac failure, cardiac dysfunction in DMD carriers needs to be examined closely and treated appropriately before the carriers become symptomatic.