Right and left isovolumic ventricular relaxation time intervals measurements were obtained as follows: from the peak R wave on the electrocardiogram to either the mitral or the tricuspid pulsed Doppler flow trace onset minus the R to end-ejection zero flow crossing of the subaortic (left side) or pulmonary (right side) D flow trace time interval. A ratio was calculated as a percent difference duration between both isovolumic ventricular relaxation time intervals. The aim was to compare isovolumic ventricular relaxation time interval values in 42 healthy controls and to study the changes induced by heart diseases in 27 patients with (1) controlled hypertension without left ventricular hypertrophy, (2) hypertrophic cardiomyopathy, and (3) Cor pulmonale. Mean values of isovolumic ventricular relaxation time intervals significantly differed at paired and unpaired studies, with right isovolumic ventricular relaxation time intervals shorter than those of the left side in all groups (p < 0.001) except for patients with Cor pulmonale. Isovolumic ventricular relaxation time intervals did not correlate with heart rate and moderately correlated with left ventricular mass and age. No significant difference was found between healthy controls and patients with controlled hypertension. Significant changes were found in patients with hypertrophic cardiomyopathy and Cor pulmonale versus healthy controls for both isovolumic ventricular relaxation time intervals. However, significant changes in the ratio were only found in patients with Cor pulmonale (p < 0.005) because of abnormal similar values for both isovolumic ventricular relaxation time intervals. This Doppler method enabled, for the first time, serial comparison of isovolumic ventricular relaxation time intervals with a homologous method. Both isovolumic ventricular relaxation time intervals significantly lengthened with age and with left ventricular indexed mass, but their ratio remained insignificantly changed except for patients with Cor pulmonale. The concomitant right and left isovolumic ventricular relaxation time intervals lengthening in patients with hypertrophic cardiomyopathy and Cor pulmonale suggests interdependence of both ventricles through the septum. This makes recommendable systematic comparison of both sides. The calculation of a ratio, free from the effect of factors intervening on isovolumic ventricular relaxation time intervals, may, in addition, be of diagnostic help.