OBJECTIVE To compare the efficacy and tolerability of Gemfibrozil and Bezafibrate in patients with hyperlipoproteinemia, types IIa, IIb and IV. METHODS In an open, randomized parallel study, the tolerability and efficacy of Gemfibrozil (G) and Bezafibrate (B) were investigated over a period of 12 weeks in 178 hyperlipidemic (HLP) patients first submitted to an 8-week "wash-out" (diet only) phase. In HLP type IIa patients, LDL cholesterol was lowered (G: -13%, B: -10%). In HLP type IIb, a decrease in triglyceride (TG) levels (G: -41%, B: -31%), an increase in HDL-cholesterol (G: +19%, B: +5%), and a decrease in the total cholesterol/HDL cholesterol ratio (G: -32%, B: -9%) predominated. Patients with type IV HLP responded to both fibrates with an appreciable reduction in TG levels (G: -45%, B: -42%). The effects of the preparations differed significantly with respect to the elevation of HDL cholesterol and a decrease in the total cholesterol/HDL cholesterol ratio in HLP type IIb patients. The respective lipid baseline concentration appeared to be of importance for the sense and extent of the changes induced.