Changes in serum lipids, apolipoproteins, and lipoproteins including high-density lipoprotein (HDL) subfractions following administration of captopril in patients with hypertension were studied. Captopril (25 mg twice daily) was administered over a 12-week period to 17 patients with mild to moderate essential hypertension. Captopril was observed to significantly reduce both systolic and diastolic blood pressure, as well as to increase HDL2- cholesterol (HDL2-C) and to decrease HDL3-cholesterol (HDL3-C); however, no significant changes in total HDL-C were recognized. Total cholesterol, low-density lipoprotein cholesterol, triglyceride, apolipoprotein (apo) A-I, apo A-II, apo B, apo C-II, apo C-III, and apo E did not change significantly. It is suggested that captopril monotherapy produces a favorable effect on HDL subfractions.