Malabsorption after total gastrectomy and Roux-en-Y reconstruction was studied in 11 patients. Absorption of fat, xylose and lactose was tested and the orocaecal transit time was radiologically determined. Bacterial colonization of the small intestine was studied by culturing jejunal juice and indirectly with a hydrogen breath test. Ten patients lost weight postoperatively and six had diarrhoea. All 11 had steatorrhoea with mean faecal fat excretion 289 +/- 55 (SEM) mmol free and esterified fatty acids/72 h (upper reference limit 60 mmol/72 h). Low xylose absorption was found in only one patient and low lactose absorption in none. The median orocaecal transit time was only 110 minutes (less than or equal to 60 min in 4 cases). Postoperative weight loss showed significant inverse correlation with orocaecal transit time. Bacterial overgrowth of the small intestine was found in four patients. The cause of malabsorption was assumed to be rapid intestinal transit in four patients and bacterial overgrowth in four others, leaving three in whom pancreatic understimulation is suggested as the reason for steatorrhoea.