The effect of cilazapril, 2.5 to 5.0 mg and hydrochlorothiazide, 25 to 50 mg, on diastolic cardiac function was studied by echocardiography and radionuclide ventriculography, using a double-blind randomized parallel-group design with a placebo run-in period, in 30 hypertensive patients. The measurements were made before and three hours after the first dose, and after ten weeks of treatment. Both drugs lowered the elevated blood pressure and did not impair the normal systolic cardiac function. In the patients treated with cilazapril an improvement in indices of diastolic function was observed after the first dose. The normalized peak filling rate significantly increased from 2.4 +/- 0.7 to 2.7 +/- 0.7 counts and time to peak filling rate was shortened from 169.4 +/- 31.0 to 151.6 +/- 40.1 msec. This improvement was sustained or even enhanced during chronic therapy. In contrast hydrochlorothiazide acutely impaired one index of diastolic function, the normalised peak filling rate, and did not alter the other indices. Long-term treatment with cilazapril, but not with hydrochlorothiazide, caused regression of left ventricular hypertrophy. CONCLUSIONS cilazapril is superior to hydrochlorothiazide in its effect on diastolic cardiac function in hypertensive patients. The beneficial effect is partially related to regression of left ventricular hypertrophy.