Thirteen adult obese subjects (12 women, one man) were placed on a low-calorie diet (240 kcal = 1.02 MJ daily; mixed protein/carbohydrate) for two weeks. Mean weight loss +/- s.d. was 7.9 +/- 1.8 kg. Total serum cholesterol and HDL-cholesterol were significantly decreased after two weeks. Their ratio was unchanged. The direction of the changes in triglyceride levels was related to the pretreatment levels, subjects with a level below 120 mg/dl showed no change or a slight increase, subjects with a level over 120 mg/dl showed a significant decrease. On a normal diet, as well as after two weeks of caloric restriction, a negative correlation existed between serum Tg and the Tg fractional removal rate (K2IVFTT). The mean K2 value increased during weight loss. On a caloric restriction the relative content of linoleic acid decreased and of arachidonic acid increased in serum cholesterol esters and phospholipids. The relative content of palmitic acid increased in CE. The fatty acid composition of Tg was unchanged.