This double-blind study compares the effects of tibric acid at four different doses to that of placebo in type IV hyperlipidemic patients. Fifty patients, divided at random in five groups of 10 patients each, received one of the following treatments: placebo, tibric acid 500 mg, 750 mg, 1,000 mg, or 1,250 mg per 24 hr. The data suggest that the 1,000 mg and the 1,250 mg doses are effective in lowering the serum triglyceride levels after 6 wk of treatment; the effect on total cholesterol is less pronouced. No effect is observed on the concentration of serum esterified cholesterol, phospholipids, and free fatty acids. It is also shown that in the dose range studied, the best fitting curve defining the four mean values of triglycerides and total cholesterol obtained with the four active treatments did not deviate significantly from linearity. Due to lack of toxicity, tibric acid may be useful in the treatment of type IV hyperlipoproteinemia.