38 obese patients with BMI in excess of 35 were monitored, the patients were divided into four groups. The first three were on a strict slimming diet which different as to the energy values (1.38 MJ--2.75 MJ) and the amount of protein (5.58 g N--17.91 g N) per 24 hours. The fourth group was on a starvation diet. Metabolic balance values and resting energy cut put were measured by indirect calorimetry during 16 days of monitoring. In all groups the body weight declined markedly by an average of 8.2-10.5 kg. Resting energy consumption dropped only during absolute fasting. The starvation diet patients exhibited a negative nitrogen balance. Positive nitrogen balance was found in groups on 11.2 g N and 17.91 g N/24 hours. Fat utilization increased and sugar utilization declined in all groups. Protein catabolism declined markedly only in the fasting patients. Summed up, the low-energy protein diet is--in comparison with absolute diet--a more physiological way to achieving weight reduction, in particular, because, despite the loss of weight, a positive weight balance is achieved, too.