Protein-substituted or protein-saving modified fasting (PSMF) is intended to prevent high protein loss during total fasting by use of low-dose substitution. 27 obese in-patients were treated over a period of four weeks with modified fasting. The daily amount of protein substituted was 33 g on an egg-white base (Modifast). As in total fasting the nitrogen balance of the first and second week was negative, balanced in the third week and positive in the fourth. The total balance over 4 weeks was negative with a nitrogen loss of 70.7 g N, equivalent to a protein loss of 442 g (lean body mass). Nitrogen and protein loss in PSMF over 4 weeks was only a third of the loss in total fasting (-183.2 g N equivalent to 1145 g protein). Protein-substituted modified fasting may be used for more than 4 weeks as nitrogen balance is stabilised as from the third week, and remained so until the end of the observation period of 8 weeks.