Hypertrophy of the left ventricle in hypertension is associated with an increased risk of morbidity and mortality. Thus, the prevention of and regression of left ventricular hypertrophy should be essential goals of antihypertensive treatment. Some commonly used antihypertensive drugs do not have this ability, despite achieving adequate blood pressure control. In a double-blind randomised parallel group study, we evaluated the effects of adjunctive long term therapy with either pinacidil or nifedipine on blood pressure and left ventricular function and anatomy (echocardiography) in 22 patients with a diastolic blood pressure above 95mm Hg who were already receiving bendrofluazide 5mg daily. Pinacidil reduced left ventricular mass from 326.3 +/- 126.2g to 251.2 +/- 114.6g (p less than 0.001) compared with nifedipine, which reduced ventricular mass from 293.6 +/- 35.7g to 267.3 +/- 34.7 (p = 0.04). In this respect, pinacidil was more effective than nifedipine (p = 0.013). Pinacidil improved left ventricular diastolic function, measured by the isovolumetric relaxation time, whereas nifedipine did not affect this parameter. Global systolic function was little affected by either drug. However, the end systolic wall stress was significantly reduced by both therapeutic regimens. Pinacidil may, therefore, be a potentially valuable antihypertensive drug.