Serum lipids, lipoproteins and apolipoproteins were measured in 16 obese female subjects before and after a short-term fast of four days. The mean body weight loss was 2.9 +/- 0.8 kg. Serum triglycerides decreased, serum, LDL, and HDL cholesterol increased significantly. Furthermore there was a significant reduction of the serum apolipoprotein B concentration, while apolipoprotein A I did not change. The atherogenic ratio apo B/apo A I were significantly lowered by the fasting period.