Despite widespread use to treat childhood hypertension, enalapril has never been studied systematically to determine effectiveness, dose response, and safety in a pediatric population. This study was conducted prospectively in 110 hypertensive children ages 6 to 16 years in two sequential phases. The primary outcome variable for both phases of the study was trough (24-h postdose) sitting diastolic blood pressure. The primary objective of the first phase of the study was to determine whether enalapril lowered blood pressure in children in a dose-dependent manner. During a 2-week, double-blind, randomized, dose-response period, patients were stratified by weight (< 50 kg or > or = 50 kg), then assigned to one of three dosing groups: low(0.625 or 1.25 mg), middle (2.5 or 5 mg), or high dose (20 or 40 mg). Reduction in blood pressure was examined as a function of dose ratio (1:4:32) and on a weight-adjusted basis. On completion of the dose-response phase of the study, patients entered a 2-week, double-blind, randomized withdrawal to either enalapril or placebo. Antihypertensive effectiveness, defined as the difference in sitting diastolic blood pressure between the placebo and enalapril groups, was determined. Adverse events were carefully recorded throughout the study. The dose-response relationship for enalapril had a negative slope and was linear over the chosen dosing range, suggesting that larger doses of enalapril were associated with a greater reduction in blood pressure. Randomized withdrawal to active drug orplacebo confirmed the antihypertensive effectiveness of enalapril in the middle- and high-dose groups. The antihypertensive effect of enalapril was maintained across age, gender, race, and Tanner stage. Enalapril appears to be an effective and generally well-tolerated antihypertensive agent in children ages 6 to 16 years. An initial dose of 2.5 mg in children weighing < 50 kg and 5 mg in children weighing > 50 kg (mean = 0.08 mg/kg) administered once daily effectively lowered blood pressure within 2 weeks in most patients. Blood pressure was reduced in a dose-dependent fashion, with larger doses resulting in a greater reduction.