The effect of Lovastatin, an HMG. CoA reductase inhibitor, on serum lipids and apolipoproteins was studied in 40 cases of primary hypercholesterolemia in a 4-month period of treatment. The level of serum lipids did not change significantly after a 35-day period of placebo treatment as compared with that of the baseline (P greater than 0.5). The patients then took Lovastatin with the evening meal in a daily dose from 20 to 80 mg for 3 months. The results were as follows: Lovastatin reduced significantly the mean serum level of total cholesterol (TC) by 31.5% (P less than 0.001), LDL-C by 39.8% (P less than 0.001), Apo-B by 27.3% (P less than 0.002), and the ratio TC/HDL-C by 35.9% (P less than 0.01). It also reduced the mean serum level of triglycerides (TG) by 22.1% (P greater than 0.05) and increased that of HDL-C by 6.3% (P greater than 0.2) and Apo-AI by 1.6% (P greater than 0.5), but without much significance. The drug was well tolerated by all the patients. Transient elevation of CPK was noticed in 2 patients and AKP in one patient. 7 patients complained of gastrointestinal discomfort. All these side effects did not necessitate stop of the medication. We are, therefore, of the opinion that Lovastatin is an effective agent for lowering the serum level of TC, LDL-C and Apo-B.