Low heart rate variability and increased QT dispersion are risk factors for cardiac mortality in various patient populations. We studied dispersion of QT interval, i.e. an index of inhomogeneity of repolarization, and heart rate variability (HRV) i.e., a measure of cardiac autonomic modulation in 76 essential hypertension cases (45 women, 53.0± 11.1 years, body mass index: 25.1±1.4 kg/m²) and 70 healthy cases (42 women, 54.0±10.2 years, body mass index: 25.5±1.6 kg/m², p>0.05). QT-corrected QT intervals and their dispersions were significantly higher in the hypertensive group (p<0.0001), all showing a direct relation with the level of systolic and diastolic blood pressures, ventricular mass index and high Lown grade ventricular rhythm problems. Time domain measures like standard deviation of RR intervals, standard deviation of the means of all corrected RR intervals calculated at 5 min intervals (p<0.0001), proportion of adjacent RR intervals differing by>50 msec (p=0.005), HRV triangular index (p =0.007), the square root of the mean squared differences of successive RR intervals (p= 0.011), and the high frequency (HF, 0.16-0.40 Hz, p<0.0001) part of the frequency domain measure of HRV were all decreased, whereas the low frequency (LF, 0.04-0.15 Hz, p=0.013) part of the frequency domain measures and LF/HF ratio (p<0.0001) were increased in hypertensive cases. Time domain and the HF part of frequency domain measures of heart rate variability showed an inverse relation with the increased levels of both systolic and diastolic blood pressures and Lown grading system of ventricular rhythm problems, whereas LF and LF/HF showed direct relations with high levels of systolic and diastolic blood pressures and high Lown grade ventricular rhythm problems. The measures of heart rate variability apart from LF and LF/HF were inversely related with the QT intervals and dispersions, whereas LF/HF was directly related with them.Therefore, we conclude that the levels of both systolic and diastolic blood pressures are related to the generation of ventricular rhythm problems either via increasing left ventricular mass which results in an increase in QT parameter measurements, or by altering heart rate variability measures indicating a disturbance in cardiac autonomic balance in essential hypertension. (Jpn Heart J 2000; 41: 173-182)