The effect of the cytotoxic drug 5-fluorouracil (5-FU) on plasma lipid levels was studied in patients and animals. Seven patients with metastatic carcinoma of the colon and three with advanced breast cancer were treated with 5-FU monotherapy by i.v. push at a dose of 500 mg/m2/d for 3-5 consecutive days. The animal group comprised 9 rabbits treated with 5-FU by i.v. push at 12-18 mg/kg/d for 2 consecutive days. Measurements of serum lipid levels were performed before and 2 and 4 weeks after 5-FU administration. No obvious change of diet, body weight and bowel habits occurred during the study period. A significant reduction of total plasma cholesterol was observed in both patients and animals. The triglyceride levels were also reduced in the rabbits. Maximal cholesterol-lowering effect was observed in patients and rabbits with higher baseline cholesterol levels. The results suggest that 5-FU might interfere with lipid metabolism.