Systolic time interval (STI) characteristics of 17 boys with Duchenne's muscular dystrophy (DMD) were compared with those of 80 normal boys who served as control subjects. The heart rate decreased linearly with age in normal control subjects (r = -.47, P less than .01). By contrast, heart rate was significantly higher in patients with DMD (P less than .001) and tended to increase further with age. Each STI variable for normal control subjects increased significantly with age (P less than or equal to .01); QII, left ventricular ejection time (LVET), and pre-ejection period (PEP), in addition, decreased with increasing heart rate (P less than or equal to .05). In dystrophic patients QII and LVET decreased with increasing heart rate (P less than .001) but were not influenced by age. None of the other STI values in dystrophic patients was significantly influenced by either age or heart rate. Mean QII, LVET, and QI were shorter and PEP, isometric contraction time (ICT), and PEP/LVET ratio were longer (P less than .001) for DMD patients than for normal control subjects. In 13/17 patients, QII and LVET were below the 95% confidence interval of the normal mean, whereas PEP, ICT, and PEP/LVET exceeded the upper limits of normal in 8, 9 and 11 patients, respectively. For dystrophic patients, the difference (delta) between the observed values and those predicted from regression equations for normal control subjects was lower for QII, LVET, and QI (P less than .01) but higher for PEP (P less than .04), ICT, and PEP/LVET ratio (P less than .001). delta QII and delta LVET increased with age (P = .001 and .032, respectively). Duchenne's muscular dystrophy is thus documented to be associated with substantial alterations in STI characteristics that suggest a compromise of global left ventricular performance. Some of these abnormalities increase with age, probably reflecting the progressive cardiomyopathy characteristics of this disease.