This study compared enalapril maleate (Enaprin) as a single antihypertensive agent and enalapril plus hydrochlorothiazide in Korean patients to determine if one regimen is superior to the other in terms of efficacy and subjective adverse effects. After a 1-week washout period, 12 patients were randomized to receive enalapril 10 mg daily and 13 patients to receive enalapril 10 mg daily plus hydrochlorothiazide 25 mg daily. After 2 weeks, enalapril was increased to 20 mg daily if diastolic blood pressure was greater than 90 mm Hg. The study period was 12 weeks. The mean changes in diastolic blood pressure in mm Hg were enalapril alone -10.7 +/- 10.5 mm Hg and enalapril plus hydrochlorothiazide -25.1 +/- 12.1 mm Hg (difference between the two groups significant at p less than 0.01). Mean dose of enalapril required to achieve goal blood pressure was 18.8 mg for monotherapy and 13.0 mg for combination therapy (p less than 0.05). Adverse effects were comparable. We concluded that enalapril in combination with hydrochlorothiazide is more effective and safe, and allows for lower dosing of enalapril than the drug as monotherapy in Korean hypertensives.