We compared the metabolic effects of 8-wk caloric restrictions with 330 or 780 kcal/d in two groups of eight obese hospitalized subjects; six control subjects were also studied. Loss of weight but not of adipose tissue was significantly greater on the 330-kcal/d diet. It is likely that dehydration rather than protein catabolism was responsible for additional loss of fat-free mass in the 330-kcal/d group because the nitrogen deficit was not excessive. The thermic response to food was blunted only in the 330-kcal/d group whereas resting oxygen uptake decreased by equal amounts in both groups. There was a decrease in 24-h urinary noradrenaline in the 330-kcal/d group but plasma fT4 was sustained when compared with the 780-kcal/d group; fT3 decreased significantly more quickly in the 330-kcal/d group. There was no correlation between plasma hormone levels and changes in oxygen uptake. Hunger scores were greater on the 780-kcal/d diet.