The effects of enalapril on plasma lipoproteins were evaluated in an open study of 12 normolipidemic outpatients with mild-to-moderate essential hypertension (World Health Organization stages I and II). After a two-week washout period, during which placebo was given, the patients received 20 to 40 mg/day of enalapril for 16 weeks. Treatment with enalapril was associated with significant increases in levels of HDL cholesterol (mean, 23%; P less than 0.001) and apoprotein A (mean, 11%; P less than 0.01), largely because of the increase in the subfraction HDL2 (mean, 43%; P less than 0.001), although the subfraction HDL3 also rose (mean, 14%; P less than 0.005). Total cholesterol and LDL cholesterol levels did not change, whereas triglycerides decreased significantly (mean, 26%; P less than 0.001). Apoprotein B was unchanged. Unlike diuretics and most beta-blockers, enalapril favorably affects plasma lipoprotein levels, thus improving the overall cardiovascular risk in hypertensive patients.