Treatment of extreme obesity with jejunoileostomy was followed by a decreased level of S-tryptophan; permanently low concentrations were recorded postoperatively in 29 out of 52 patients. Patients in the low tryptophan group had a higher rate of weight loss and a hgiehr incidecnce of electrolyte disturbances and signs of liver injury. Symptoms of depression and anxiety were slightly more common in patients with low S-tryptophan. The influence of a decreased S-albumin and a deranged amino acid pattern on the non-protein-bound fraction of S-tryptophan needs further investigation. Serum levels of tryptophan rose significantly after rwo weeks' oral supplementation with 1.2 g L-tryptophan daily; this dosage was insufficient to normalize a low S-tryptophan level in patients who have undergone jejunoileostomy.