Patients with familial hypercholesterolemia (FH) have had a life-long sustained elevation of low-density lipoprotein (LDL) cholesterol levels. Consequently, there is a need to maximally lower their elevated levels, and this usually requires lowering LDL levels more than 50%. Because no single hypolipidemic drug will consistently produce such degrees of lowering, combination drug therapy with two or even three agents is required to produce the desired degree of cholesterol lowering. A prospective trial was designed to determine if combination therapy using three hypolipidemic agents could effectively lower LDL levels in 17 severely affected FH subjects. Colestipol hydrochloride (10 g b.i.d.), probucol (500 mg b.i.d.), and lovastatin (20 or 40 mg b.i.d.) were given to each patient, in varying combinations, over a 25-month period. Lovastatin (40 mg/day) uniformly lowered LDL levels 36%. Probucol lowered LDL only 14% and in a variable manner. The combination of lovastatin and probucol lowered LDL no better than lovastatin alone. Lovastatin plus colestipol lowered LDL 52%; probucol added as a third agent produced no further lowering. Lovastatin (80 mg/day) plus colestipol lowered LDL 56%. Lovastatin increased high-density lipoprotein (HDL) cholesterol levels 6%, whereas probucol decreased HDL 29%. In all patients there was an effective lowering of LDL levels, ranging from 40% to 70%. Thus, lovastatin plus colestipol is an effective hypolipidemic regimen for producing marked decreases in LDL levels in FH subjects. The addition of probucol as a third hypolipidemic agent adds little to the therapeutic regimen as measured by lowering of LDL levels.