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In a double-blind randomised parallel trial, diltiazem (120mg 2 or 3 times daily) was compared with hydrochlorothiazide/triamterene (25/50 or 50/100mg once daily) for 12 weeks, with regard to blood pressure lowering and adverse effects, in 61 evaluable patients with mild to moderate essential hypertension (diastolic blood pressure ⩾ 95mm Hg). The two treatments were equally effective in reducing blood pressure, and the proportion of responders did not differ. Combined therapy (diltiazem + hydrochlorothiazide/triamterene) was begun in the ensuing 16 weeks (i.e. single-blind phase) in those patients where a diastolic blood pressure of ⩽ 90mm Hg (or a decrease of ⩾ 10mm Hg for those with a baseline of 95 to 99mm Hg) was not achieved. In this instance, a further relevant lowering of blood pressure occurred. An analysis of all patients (monotherapy and combined therapy) showed that goal blood pressure was achieved (p = 0.06) in more patients treated initially with diltiazem (22 of 28 patients, 78.6%) than with hydrochlorothiazide /triamterene (18 of 33 patients, 54.6%). Final mean daily doses were diltiazem 287mg, hydrochlorothiazide/triamterene 28/76mg, and combined therapy 300 + 37.5/75mg, respectively. The incidence of adverse events during treatment with hydrochlorothiazide/triamterene was almost double that observed with diltiazem (46 vs 24%). Headache was the most common complaint in both groups (17 and 14%, respectively). Oedema was also frequent with diltiazem therapy (10.3%), while nausea (11.4%), nervousness (8.6%), decreased libido (8.6%), and weakness (8.6%) were reported only with hydrochlorothiazide/triamterene. Combination therapy was associated with significantly lower mean serum potassium levels and significantly higher mean uric acid levels than was diltiazem monotherapy. In conclusion, diltiazem is as effective as hydrochlorothiazide/triamterene in reducing blood pressure but produces fewer side effects. Combining the 2 treatments results in additional blood pressure reduction. To establish the long term usefulness of diltiazem, it will be essential to evaluate its ability to reduce the morbidity and mortality associated with hypertension, especially the development of congestive heart disease.