Bezafibrate, a new hypolipidemic agent, was evaluated in a single blind, placebo-controlled study lasting 14 months to 3 years in 40 patients with primary hyperlipoproteinemia of various types (23 patients with type II, 15 with type IV and 2 with type V). Bezafibrate in a dose of 200 mg 3 times daily reduced total serum cholesterol by an average of 17% (p less than 0.001) in hyperlipoproteinemia type II and by 24% (p less than 0.001) in type IV or V, lowered serum triglycerides by 31% (p less than 0.001) in hyperlipoproteinemia type II and by 58% (p less than 0.001) in type IV or V, while high-density-lipoprotein-cholesterol was increased by 17% (p less than 0.001) in type II and by 36% (p less than 0.001) in type IV or V. The few side effects encountered were only mild and transient. These findings show that bezafibrate affords safe and effective long-term treatment of patients with hyperlipoproteinemia.