The causes of fat malabsorption after gastric resection were studied in patients randomized to Billroth I gastroduodenostomy or Roux-en-Y gastrojejunostomy after antrectomy and selective gastric vagotomy for intractable prepyloric peptic ulcer. Eighteen patients were studied 2 years or more postoperatively. Assessment of food intake concerning protein, fat, carbohydrate, vitamins and minerals showed deficiency in seven cases, but without significant difference between the two operation types. Postoperative weight loss did not differ significantly between the two study groups, but the faecal excretion of fat was significantly larger (p less than 0.05) after gastrojejunostomy than after gastroduodenostomy. Since both the oro-caecal transit time and the breath excretion of hydrogen after a standardized test meal showed no intergroup difference, the results suggest that duodenal exclusion may be of particular pathophysiologic importance for the observed malabsorption of fat after gastric resection.