In 16 obese male patients lipid and apolipoprotein A-I and B in serum were assessed before and after weight reduction. The mean age of the subjects was 34.7 +/- 2.7 years, the mean body weight 124.4 +/- 4.0 kg and the mean BMI 39.1 +/- 1.5 kg/m2. After 10 weeks on a low-energy diet of 2500-6000 kJ/day a decline of body weight was observed on average by 14% of the initial value. The total triacylglycerolaemia declined by 46% (p less than 0.02). The total cholesterol level declined by 12% (ns), HDL-cholesterol by 7% (ns), LDL-cholesterol by 8% (ns). The apo A-I did not change during weight reduction, apo B declined by 13% (ns). The ratio of apo B to apo A-I declined by 12.5% (p less than 0.05). The presented results indicate that after weight reduction of obese patients by a low-energy diet the risk of atherosclerosis declines as the apo B/apo A-I ratio declines.