Absorption studies were performed in 17 patients with ulcerative colitis operated on with colectomy and an ileal two-limbed J-pouch anastomosis. The patients were studied 3 and greater than or equal to 18 months after closure of the temporary ileostomy. Increased stool mass (median, 609 g/24 h) was found in all patients and was unchanged with time. Moderate steatorrhoea was present in 29% of the patients 3 months postoperatively, but faecal fat excretion normalized with time. Calcium absorption was normal in all but one patient regardless of time after operation. An abnormal bacterial deconjugation, evaluated by a 14C-glycocholic acid breath test was present in 27% of the patients and increased significantly with time. Forty per cent of the patients had increased faecal bile acid excretion. B12 malabsorption was present in 29-35% of the patients. In conclusion, ileal J-pouch anastomosis for ulcerative colitis causes increased stool mass in all patients and produces moderate bile acid deconjugation and malabsorption in about one-third to half. Substitution therapy with vitamin B12 is necessary in about one-third of the patients. Intestinal adaptation as far as absorption is concerned is minimal after the first 3 postoperative months.