Bacterial contamination of the small bowel is probably the commonest cause of occult malabsorption in the elderly. It may occur in patients without a 'blind loop' or suggestive symptoms of diarrhoea and weight loss. We have prospectively studied the apparent prevalence of presumed bacterial contamination of the small bowel and its effect on nutritional state. Subjects were divided into three groups: (A) 54 young fit subjects; (B) 103 fit community elderly subjects; (C) 73 elderly long-stay hospital patients. All subjects had simultaneous lactulose hydrogen breath test and 14C-glycocholic acid breath test. Nutritional state was assessed by anthropometry, haematology and biochemistry. There were significantly fewer positive 14C-glycocholic acid breath tests in the young than in the elderly subjects (3% vs. 20% and 17%, p less than 0.0001) but no difference in the number of positive hydrogen breath tests between groups. There was no association between positive breath tests and anthropometry, haematology and biochemistry except for a lower albumin in group B and a lower red blood cell folate in group C with positive breath tests. These abnormal breath tests indicate that bacterial contamination of the small bowel may be common in normal fit elderly people and in elderly long-stay hospital patients and may be a concomitant of 'normal' ageing, not necessarily leading to ill-health.