The aim of the study was to evaluate whether the combination of ketanserin with captopril exerts an additive antihypertensive effect, as compared with single drug treatment. Twelve patients with uncomplicated moderate essential hypertension received, according to a randomized, double-blind, crossover design, ketanserin (40 mg twice daily), captopril (50 mg twice daily), the combination of the two drugs at these dosages, and the corresponding placebo, each treatment being given for 1 month. Both ketanserin and captopril as monotherapy similarly and significantly reduced blood pressure as compared with placebo (p less than 0.001). The combination treatment of ketanserin plus captopril further and significantly reduced blood pressure when compared with single drug treatment (p less than 0.001). Moreover, the percentage of responders and patients whose blood pressure was normalized were significantly greater under the combined treatment than under ketanserin or captopril monotherapy (p less than 0.001). These data indicate that the combination of ketanserin plus captopril exerts a clear additive antihypertensive effect when compared with each treatment as monotherapy, a finding that suggests this combination can be usefully employed in the treatment of hypertensive patients.