The effect of five weeks fasting on body weight, lean body mass and fat weight was studied in 14 obese subjects (BW 92-200 kg, Broca's index 1.3-2.4). Before and after treatment the patients were hospitalized and put on a standardized weight maintaining diet, and variables reflecting thyroid function were compared before and after weight reduction. There was a considerable variability in the reduction of body weight (4-22 kg), lean body mass (1-7 kg) and fat weight (4-18 kg). None of the thyroid variables were correlated to the degree of obesity. The body weight loss during treatment was correlated to initial body weight, initial lean body mass and initial body fat weight. The reduction in body fat, however, was significantly correlated only to initial body weight (r=0.70) and lean body mass (r=0.61). Plasma concentrations of thyroxine, triiodothyronine and reverse triiodothyronine were not related to reduction of body fat, whereas the T3 uptake test was significantly correlated to the fat weight loss (r = 0.67). Multiple regression analyses demonstrated in the initial body weight and T3 uptake were independently correlated to fat weight reduction. Together these variables accounted for about 75 per cent of the variability in fat weight reduction during fasting therapy. Plasma concentrations of rT3 before treatment was significantly correlated to the reduction lean body mass (r=0.78), whereas rT3 was not related to fat weight reduction (r=0.28). The determination of thyroid hormones and T3 uptake seems to have prognostic value for the effect of caloric restriction on different metabolic compartments.