The purpose of this study was to investigate the cardiorespiratory function in Duchenne (DMD) and Becker muscular dystrophy (BMO) patients and to determine whether there is a correlation between these functions and muscular strength. The study involved 32 patients with progressive muscular dystrophy (28 DMD and four BMD). The mean age of the patients was 9.6 +/- 3.5 years. Cardiac investigations were performed in all of the patients, and pulmonary function tests were obtained in 16 cases. In five cases (31%), vital capacity (VC) was less than 80 percent of the predicted value. There was a good correlation between VC and muscular strength. There were various cardiologic findings in 50 percent of the cases with DMD. Electrocardiographic changes were present in 43 percent of the patients. Left ventricular systolic function in the patients who could not walk was significantly lower than that of the patients who could walk. There may be some unknown mechanisms that preserve left ventricular function relatively in the normal range in spite of cardiac involvement.