The effects of fenofibrate on lipids, lipoproteins, and apolipoproteins in 33 subjects with primary hypercholesterolemia were assessed in a 6-month parallel group study, placebo (n=15) versus fenofibrate 300 mg/day (n=18), followed by an open label 6-month treatment period. After stabilization on an isocaloric low fat (<35% total calories) diet with <250 mg cholesterol/day and a P/S ratio of 1, and maintenance of LDL-cholesterol (LDL-C) levels .gtoreq. 175 mg/dl, subjects received placebo for 6 weeks and were then randomized into placebo or fenofibrate groups for 6 months, followed by open label treatment for 6 months. During the 6-month double-blind period, compared to the placebo group, the treatment group had significant reductions in total cholesterol, LDL-C, total apo B, and triglyceride, and increments in HDL-cholesterol, apolipoprotein A-I and apolipoprotien A-II (P < 0.01 for all comparisons). Compared to placebo baseline, therapy with fenofibrate resulted in a reduction of LDL-C, apo B, and the LDL-C/HDL-C ratio of 15%, 13%, and 18% respectively; HDL-C, Apo A-I, and apo A-II increased respectively 12%, 13% and 30% (P < 0.01 for all comparisons). Mean adherence during the double blind phase of the trial was 95% in the drug group and 96% in the placebo group. An additional 6 months of open label fenofibrate therapy maintained the reduced total and LDL-C as well as the elevated HDL-C, apo A-I and apo A-II in the drug-drug group. Similar lipid, lipoprotein, and apolipoprotein changes were achieved in the placebo-drug group, so that the two sequence groups did not differ at the end of 6 months of open label therapy, P > 0.1. In subjects with primary hypercholesterolemia, fenofibrate, in conjunction with diet, effectively lowered LDL-C, apo B, and triglycerides, and increased HDL-C, apo A-I, and apo A-II. These effects, combined with its ease of administration and lack of side effects, enhance fenofibrate's importance and utility as a pharmacologic agent in the treatment of hypercholesterolemic subjects.