In treating 40 obese children aged 8--15 years with the I--III degree of constitutional-exogenous adiposity a hypocaloric diet with a reduced content of fats and carbohydrates was applied. The effect of the treatment was evaluated by determining the body mass, the characteristics of the lipids metabolism, e.g. by measuring the total lipids, tryglycerides, cholesterol, the blood plasma lipoproteins, as well as the fatty acids blood plasma composition and of the erythrocytes stroma before and after the treatment. The coefficient of the food fatty acids metabolization effectiveness into polyunsaturated fatty acids (CEM), e.g. the proportion of the arachidonic acid in the sum-total of other polyunsaturated fatty acids with 20 and 22 carbon atoms, was calculated. The CEM magnitude with the applied treatment was found to change variously, i.e. either rising or falling or remaining unchanged. It is presumed that the unequal orientation of the CEM changes in treating obese changes reflects a different degree of the profoundness of changes of the food fatty acids metabolism occurring in them. The value of the CEM application for prognosis of adiposity and evaluating the effect of the treatment in each individual case is stressed.