Metformin's hypolipidemic effects (2.55 g/day for 3 months) have been studied in 19 subjects with Fredrickson's Type IV hyperprebetalipoproteinemia. The majority of patients were above ideal body weight (relative body weight = 118 +/- 2.7 %). Eleven of the subjects presented chemical diabetes, 5 fasting hyperglycemia, and 3 normal glucose tolerance. After treatment with metformin, body weight showed a slight, but significant reduction (--2.4 +/- 0.3 kg). Glucose tolerence was not substantially altered while basal glucose was significantly reduced in the 5 subjects with fasting hyperglycemia. Basal plasma insulin was significantly reduced in all the patients following metformin treatment. Insulin response to OGTT was slightly reduced in the subjects with fasting hyperglycemia. Independent of the patients' glucose tolerance, metformin treatment induced a marked decrease in plasma triglycerides (-- 40 %) and a reduction in plasma cholesterol (-- 12 %). No correlation was found between triglyceride and cholesterol reduction and body weight, glucose, and plasma insulin variations. Like phenformin, metformin acts not only on glucose metabolism and insulin secretion but on lipid metabolism as well.