This prospective randomized study involved 28 patients with moderate essential hypertension who for one year, took either hydrochlorothiazide [Esidrex (E) 25-50 mg per day] or a combination of altizide 15 mg--aldactone 25 mg [Aldactazine (A) 1 or 2 tablets per day] without potassium supplements. Blood pressure, plasma potassium, exchangeable potassium, ventricular premature contractions measured by Holter and plasma magnesium were monitored. Blood pressure was brought to normal in both treatment groups (p < 0.001). Plasma potassium fell by 0.19 mmol/l with A (NS) and 0.35 mmol/l (p < 0.01) with E. Changes in potassium pool were not significant. There was a non-significant reduction in the number of ventricular premature contractions in both groups. There was no correlation between the few cases of frank hypokaliema, fall in potassium pool and complex ventricular premature contractions. A review of the literature offers no solid arguments suggestive of significant potassium risk associated with these diuretics. Any such risk can be eliminated by the use of low doses, combined if necessary with a potassium-sparer or a hypotensive agent of another group.