Thirty-two patients with diarrhoea, on average four years following truncal vagotomy and drainage, were studied. A comparison was made with 24 patients without postvagotomy diarrhoea. The incidence of bacterial colonization of the upper small intestine was no different in the two groups, though patients with a gastroenterostomy had a significantly higher incidence than those with a pyloroplasty. There was a higher incidence of ;anaerobic colonization' in patients with diarrhoea, but statistical significance was not reached. Colonization was associated with significantly lower levels of gastric acid secretion. Though 13 patients with diarrhoea had an abnormal faecal fat excretion, no correlation could be found between this and the severity of the diarrhoea or bacterial colonization, either with an anaerobic or a coliform type flora.In patients with diarrhoea, no small intestinal mucosal abnormality was detected, the mean haematological and serum biochemistry values were within normal limits, and the body weight was similar to that before operation. Two patients with diarrhoea had abnormal haematological values five years following vagotomy and gastroenterostomy in association with ;anaerobic colonization' of the upper small intestine. As the incidence of haematological abnormalities after gastric surgery increases with time, colonized patients might merit particularly close clinical observation.