In 287 obese men and women with BMI values of 29-50 kg/m2, divided by sex and age, the effect of a low-energy diet on triacylglycerol (TG), total cholesterol (TC) and HDL-cholesterol (HDL-C) serum levels was investigated. On a diet of 2500 kJ/day (4 weeks) and 4000-7000 kJ/day (6 weeks) the body weight declined by 10-11% from the initial value. There was a significant drop of TG in obese men above 25 years and in obese women above 35 years of age. The mean TC values did not change significantly after weight reduction. The mean HDL-C levels increased significantly after weight reduction, as compared with initial values in both sexes in the groups aged 26-35 years. In this group there was also an increase of the relative ratio of HDL-C in TC and the value of the atherogenic index declined. The authors draw attention to the fact that with regard to preserving normal blood lipid levels, in particular in obese men, it is necessary to reduce or normalize body weight in the course of the third decade.