The efficacy and tolerance of 750 mg of Acipimox was tested in 38 pts with primary dyslipidemias: 20 type IIa, 12 type IIb, and 6 type IV. All pts had been poor responders to a 2 month diet according to the recommendations of the National Cholesterol Education Program. Clinical examination, eye fundus, and the following laboratory tests: total cholesterol (TC), HDL, triglycerides (TG), total bilirubin, alkaline phosphatase, oxalacetic and pyruvic transaminases, uric acid, plasmatic creatinine, albumin, postprandial glucose test, hematocrit, white blood and platelet count were performed 60 days before drug initiation, 60 and 180 days after treatment had been started. No side effects were observed (myositis, visual gastrointestinal). 50% of the pts had slight to moderate flushing which appeared the first 3 days and lasted 14 +/- 7 days after treatment had been started. Plasmatic creatinine increased from 0.89 to 1.86 mg/dl in pt with one kidney, returning to normal levels 30 days after Acipimox interruption. After 180 days of therapy in the IIa group TC was -27% (p < 0.001), HDL + 15% (p < 0.001); in the IIb group: TC-23% (p < 0.001), HDL +9% (NS), TG -48% (p < 0.001); and in the IV group: TC-10% (p < 0.05), HDL +20% (p < 0.001), TG-53% (p < 0.001). Acipimox is well tolerated and is useful as a lipid-lowering drug in type IIa, IIb and IV dyslipidemias. Further studies are necessary to clear effects of the drug on renal metabolism and on long term survival of coronary pts.