Systole and diastole are the fundamental periods of the cardiac cycle, yet little emphasis has been placed on their relative duration when evaluating heart failure. Cardiac intervals are used to assess ventricular function, but the relative duration of systole and diastole for defining function have not been evaluated. We hypothesized that in heart failure, systole is prolonged and diastole shortened. We defined systole and diastole in 16 children with idiopathic dilated cardiomyopathy and in 16 normal controls, matched for age and gender, using the mitral regurgitant (MR) and tricuspid regurgitant (TR) flow duration. The systole and diastole durations (expressed as a fraction of the cardiac cycle) were correlated with heart rate and age and compared between groups. The subjects were compared with gender- and age-matched controls (9.98 ± 6.1 vs 9.88 ± 6.08 years, p = NS). The 2 groups had similar heart rates (104 ± 31 vs 92 ± 34 beats/min, p = NS). The systole duration was not significantly different when measured by MR versus TR duration (0.60 ± 0.10 vs 0.57 ± 0.11 of the cardiac cycle, respectively, p = NS). Systole was prolonged in subjects compared with controls (0.60 ± 0.1 vs 0.42 ± 0.08, respectively, using MR duration, p <0.0001 and 0.57 ± 0.11 vs 0.41 ± 0.07, respectively, using TR duration, p = 0.0008). The systolic/diastolic ratio was 0.77 ± 0.24 in the controls versus 1.57 ± 0.98 in the patients with idiopathic dilated cardiomyopathy using the TR duration (p <0.005) and 1.67 ± 0.68 using the MR duration (p <0.0001). The systole duration correlated with heart rate in subjects (r = 0.79, p = 0.0003) and controls (r = 0.69, p = 0.003). In conclusion, systole is significantly prolonged and diastole correspondingly shortened in heart failure. Reversal of the normal systolic/diastolic ratio may compromise cardiac filling and function. The systole and diastole duration are easily measured using routine Doppler flow, enhancing assessment of cardiac function in children.