Low calorie diet (4.18 MJ daily) was given to 48 (35 female, 13 male) obese patients during 24 days followed by 7 days of normal diet (9.61 MJ daily). Patients were treated in sanatorium under steady medical control. The average body weight decrease in women was 5.3 kg and in men 6.2 kg. A significant decrease in serum triglycerides, total cholesterol and apolipoprotein B in low density lipoproteins (LDL-apo B) was observed after low calorie diet. No influence on high density lipoprotein cholesterol (HDL-chol), HDL2-chol and HDL3-chol in men and on HDL3-chol in women was observed. The HDL2-chol decreased in women. The LDL-apo B/HDL-chol ratio decreased in both sexes, however, it was significant only in women. After 7 days of normal diet no further significant changes in serum lipids and lipoproteins were observed.