Patients with mild to moderate essential hypertension were treated in four centers with hydrochlorothiazide (HCTZ) 25 mg daily for 4 weeks. Those patients failing to achieve control (supine diastolic blood pressure (DBP) 90 mm Hg or lower) were randomly assigned to nitrendipine (NTP) 5 mg or placebo (PLA) b.i.d. NTP or PLA was increased stepwise to 20 mg b.i.d. as needed to achieve control. Treatment with HCTZ was maintained at 25 mg daily. Eighty-six patients completed the study: 41 in NTP + HCTZ and 45 in PLA + HCTZ groups. Both groups were comparable in baseline characteristics. Supine systolic blood pressure (SBP) was 148.6 ± 14.4 and DBP was 96.0 ± 5.4 mm Hg in the NTP + HCTZ group. SBP was 147.4 ± 16.6 and DBP was 96.4 ± 5.6 mm Hg in the PLA + HCTZ group. At the end of 7 weeks of titration and treatment with NTP or PLA combined with HCTZ, SBP had fallen to 133.4 ± 15.1 and DBP to 85.4 ± 5.9 mm Hg in the NTP + HCTZ group. SBP fell to 138.8 ± 16.2 and DBP to 90.7 ± 6.3 mm Hg in the PLA + HCTZ group. The increment in lowering of both SBP and DBP was significantly greater (p = 0.002 and p = 0.0007, respectively) in the NTP + HCTZ group compared to the PLA + HCTZ group. © 1984 Raven Press, New York.