Total fasting was compared with VLCD (1.28 MJ, 300 kcal/day; 56 g protein, 12 g CHO) in 14 euthyroid obese patients, selected as matched pairs, over a period of 28 days. The weight loss was significantly greater during fasting than during the VLCD (16.5 kg vs. 12.7 kg). The basal metabolic rate of (BMR) showed a significant decrease (25 per cent) during total fasting, but was unchanged with the VLCD. With both diets there was a transient increase in T4, FT4 and FT4I, and a tendency for TSH to decrease. There was a decrease in T3 and a transient increase in rT3. Nitrogen balance was attained in five of seven patients on the VLCD after four weeks.