Serum concentrations of free triiodothyronine (T3) and total T3 were measured in 25 grossly obese patients with constant body weight and no dietary restrictions. The results were compared with serum free T3 and total T3 in 55 non-obese volunteers matched in sex and age. Median serum concentration of free T3 (4.5 pmol/l) and total T3 (1.80 nmol/l) in the obese patients were not significantly (P greater than 0.10) different from free T3 (4.6 pmol/l) and total T3 (1.84 nmol/l) in the normal persons. The risk of type II error (P beta) for overlooking a possible, true difference between obese and normal persons were estimated. If there is a difference in the mean value of free T3 then it is less than 0.5 pmol/l (P beta less than 0.05), corresponding to 12 per cent of the mean value of free T3 in the normal persons. If there is a difference in the mean value of total T3 then it is less than 0.21 nmol/l (P beta less than 0.01) corresponding to 11 per cent of the mean value of total T3 in the normal persons. The caloric and carbohydrate intakes were estimated in 17 of the obese patients and showed no significant correlations with serum free or total T3. The study indicates that the thyroid function per se is normal in obesity and adapts to increasing body weight by enhanced production of thyroid hormones and thereby sustaining a normal serum concentration of free T3.