In 12 patients with primary type IV hyperlipoproteinemia the lipid lowering effect of constant doses of the biguanides metformin, phenformin and buformin was compared. After a 2 months dietary period bodyweight and initial triglyceride and cholesterol plasma concentrations were stabilized. The three biguanides were given for two months each, and this 6 months drug period was followed by another dietary period. While, 0.15 g phenformin and 2.55 g metformin reduced triglycerides by about 50% the reduction by 0.30 g buformin was only half. This was due to 4 nonresponders with initial triglycerides of more than 700 mg/100 ml. In contrast the triglyceride lowering effect of the other two biguanides was positively correlated to the pretreatment triglycerides. The cholesterol lowering effect of biguanide treatment was far less pronounced. There were no significant alterations of body weight and no influence on basal insulin and glucose concentrations. In conclusion, all three biguanides tested in a dosis of 3 tablets each are effective in lowering triglycerides although differing in efficiency and depending from the degree of typ IV hyperlipoproteinemia.