The purposes of this study were to: (1) evaluate the progression of cardiac involvement in Duchenne's muscular dystrophy using systolic time intervals (PEP/LVET); (2) determine if the degree of cardiac involvement bears a relation to the severity of skeletal muscle disease; and (3) describe the M-mode and two-dimensional echocardiographic findings. In 1970, systolic time intervals were studied in 16 patients. During the 10-year interim, two patients were lost to follow-up study, and five patients died. Nine remaining patients were re-studied in 1980 with M-mode and two-dimensional echocardiography as well as systolic time intervals. The PEP/LVET value of these nine patients increased from 0.37 +/- 0.05 (mean +/- SD) in 1970 to 0.47 +/- 0.07 (p less than 0.005) in 1980. Three patients remained ambulatory, and their PEP/LVET value (0.41 +/- 0.04) was significantly better than that of the nonambulatory patients (0.50 +/- 0.07, p less than 0.05). The M-mode echocardiography percentage diameter change was also worse in the nonambulatory group (21 +/- 4 percent versus 34 +/- 7 percent, p less than 0.02). The five patients who were nonambulatory in 1970 died in the intervening 10 years. This study demonstrated that the heart disease of Duchenne's muscular dystrophy is progressive and that the severity of skeletal muscle disease is probably associated with the degree of cardiac dysfunction.