This study was conducted to assess the therapeutic utility of combining amlodipine with captopril in patients with moderate-to-severe hypertension. Patients had hypertension of WHO grades I-III, with initial mean sitting and standing diastolic blood pressure of 100-119 mm Hg (phase V) after 2-4 weeks on placebo, and had remained uncontrolled (diastolic blood pressure > 95 mm Hg) despite a further 4 weeks on low-dose captopril. Twenty-nine patients entered the computer-randomized, double-blind, placebo-controlled, 2-way crossover comparison of either amlodipine 10 mg once daily or matching placebo added to continued therapy with captopril 25 mg twice daily for 4 weeks. Patients then acted as their own control and received the alternative amlodipine/placebo treatment plus their continued captopril therapy for another 4 weeks. Once-daily amlodipine was shown to be effective when combined with captopril. Mean baseline supine systolic blood pressure decreased from 167 to 149 mm Hg and standing systolic blood pressure from 167 to 144 mm Hg. Mean supine diastolic blood pressure decreased from 105 to 92 mm Hg, and standing diastolic blood pressure decreased from 110 to 96 mm Hg. The placebo-corrected amlodipine differences in mean changes from captopril baseline were -18/-12.2 mm Hg for supine and -20.1/-11.9 mm Hg for standing systolic and diastolic blood pressures, respectively (p < 0.001 for all 4 measurements). The most common side effects encountered with amlodipine were flushing and pedal edema. The combination of amlodipine and captopril was well tolerated, and no patient discontinued therapy. No significant treatment-related effects on biochemical and hematologic parameters were noted.