Whether a diuretic confers an additional antihypertensive effect on patients receiving treatment with calcium channel blocking drugs remains controversial. The results of seven recently reported trials are summarized, with the conclusion that much of the confusion arises because of inadequacies in trial design, particularly the failure to take into account placebo responses and period effect, and the inclusion of too few subjects to give the trial sufficient power to demonstrate a significant fall in blood pressure. Further difficulties in comparing one trial with another include differences with respect to age, race, severity of hypertension and renin status in the patient populations under investigation, since all these factors may influence responsiveness to therapy with both classes of agent. Although there may be theoretical grounds for believing that, under circumstances of optimal calcium entry blockade in vascular smooth muscle, diuretics might not cause any further reduction in blood pressure, until such time as full dose-response relationships are established for each class of agent alone and in combination, the problem will remain unresolved.