Simvastatin (MK733), derived from lovastatin by substituting CH3 for H at the 2' position, is a potent hydroxymethylglutaryl coenzyme A (HMG CoA) reductase inhibitor. Its cholesterol-lowering effect in 40 patients with heterozygous familial hypercholesterolaemia was more pronounced (an LDL-cholesterol reduction of 43%) than that of cholestyramine monotherapy in a matched group of 20 patients (30% reduction). The combination of the 2 drugs for 50 patients who tolerated cholestyramine was even more effective (a 54% reduction of LDL-cholesterol). The other changes were as follows: total cholesterol (simvastatin [S] -36%, cholestyramine [C] -23%, both drugs in combination [S + C] -45%); HDL-cholesterol (S +16%, C +9%, S + C +20%); triglyceride (S -21%, C +11%, S + C -17%); and the apolipoprotein B/apolipoprotein A ratio (S -51%, C -39%, S + C -67%). Cholestyramine caused more gastrointestinal adverse effects (12 of 20 patients), whereas a transaminase increase was seen both with cholestyramine (2 of 20 patients) and simvastatin (3 of 40 patients) and with the combination (6 of 50 patients). Treatment with simvastatin decreases the atherogenic potential of plasma more than cholestyramine monotherapy and causes fewer adverse effects. For those patients who tolerate cholestyramine, the combination of the drugs is even more potent.