Plasma lipids, lipoproteins and apolipoproteins were studied before and during 6 months of pravastatin administration in patients with hypecholesterolaemia. After a 1 month placebo run-in period, the patients were treated double-blind either with placebo (n = 25) or with pravastatin (n = 25) for 6 months. Placebo or pravastatin 10 mg during the first month, 20 mg during the second month and 40 mg during the additional 4 months was administered once daily in the evening. Compared with the placebo group the plasma concentration of total cholesterol and phospholipids, free cholesterol and cholesterol esters as well as the plasma LDL-cholesterol and LDL-phospholipids were decreased during 6 months of pravastation therapy. No changes in plasma VLDL-, HDL-, HDL2-, or HDL3-cholesterol, -phospholipids or -triglycerides were observed in the pravastatin-treated patients. A decrease in the plasma level of apolipoprotein B and of LDL-apo B, but not of VLDL-apo B, was observed during pravastatin therapy; the plasma apolipoprotein AI and AII levels as well as HDL2- and HDL3-apo AI and apo AII levels remained, however, unchanged. Plasma lipoprotein Lp(a) did not change during pravastatin therapy whereas the plasma lecithin cholesterol acyltransferase activity (LCAT) increased. In conclusion, treatment of hypercholesterolaemic patients with pravastatin results in a decrease in the plasma concentration of total and free cholesterol, LDL-cholesterol, apolipoprotein B, LDL-apo B, phospholipids and cholesterol esters and in an increase in plasma LCAT activity. Plasma Lp(a), HDL-cholesterol and triglyceride levels remained, however, unchanged.