The purpose of this study was to evaluate the potentiating effect of hydrochlorothiazide (HCTZ) 12.5 or 25 mg once daily when added in a placebo-controlled double-blind randomized study of patients with essential hypertension, whose diastolic blood pressure (DBP) was not adequately controlled (DBP > 90 mmHg) following 6 weeks of single-blind treatment with the angiotensin converting enzyme (ACE) inhibitor enalapril, 20 mg once daily. Forty-eight patients started the first period with enalapril after 4 weeks on placebo. In 13 patients DBP fell to < or = 90 mmHg after enalapril for 6 weeks. In this group supine mean arterial pressure (MAP) was reduced by 13% (P < 0.01). In the patients whose DBP was > 90 mmHg after 6 weeks on enalapril (n = 32) the average supine MAP fell by 9% (P < 0.001). After 3 weeks there was no further drop in blood pressure (BP). Addition of HCTZ to the 32 patients with DBP > 90 mmHg caused a significant further drop in supine BP by 13/7 mmHg with 12.5 mg and by 15/7 mmHg with 25 mg. Seven patients discontinued the study, none due to side effects on enalapril alone. Well-being, assessed with a special questionnaire, was significantly better with enalapril as monotherapy compared with previous treatment, but not different from well-being during the placebo periods. It is concluded that 20 mg enalapril once daily lowered BP effectively and was well tolerated. The maximum BP lowering effect was seen within 3 weeks. Addition of HCTZ caused a significant, and clinically relevant, further drop in BP.(ABSTRACT TRUNCATED AT 250 WORDS)