To compare two popular strategies for intensifying treatment for hypertension, a double-blind, randomized, prospective, parallel-group, and partial crossover study was done. After 2 weeks of placebo run-in (baseline) and 3 weeks of 5 mg enalapril once daily, 217 patients were randomized to 6 weeks of treatment with either a low-dose combination therapy (5 mg enalapril + 5 mg felodipine ER once daily, Lexxel, Astra Merck, Inc.), or a higher dose of monotherapy (10 mg enalapril once daily, Vasotec, Merck & Co., Inc.). The group randomized to the combination had significantly greater reductions in sitting systolic/diastolic blood pressure (BP)--14.2/10.6 mm Hg compared with baseline versus 9.6/7.4 mm Hg (P < .05/.01)--as well as a greater percentage of patients having achieved either diastolic BP < 90 mm Hg or a decline of at least 10 mm Hg (responders), 59% v 41% (P < .01). When patients originally taking 10 mg enalapril were crossed over to the combination therapy for a further 6 weeks, there was a further BP reduction and increase in response rate, with loss of significant differences compared with those treated continuously with the combination for the entire 12 weeks. The greater BP-lowering efficacy of the combination was independent of age, gender, and race. There were no significant differences in tolerability between the regimens. These data support the hypothesis that in patients who do not achieve goal BP reduction with a low dose of an antihypertensive agent, a combination of two drugs with complementary mechanisms of action is more effective than increasing the dose of the first agent.