The pathogenesis of malabsorption has been studied in 70 patients who presented over the age of 65 years and who were referred to a special investigative unit. Often more than one cause was apparent. Fourteen patients had pancreatic insufficiency, most of whom had no history of pain, alcoholism or gallstones. Twenty-three patients had the postgastrectomy syndrome or small-bowel diverticulosis or both. There were eight coeliacs aged 65-72 years at diagnosis. Fifteen patients had an anatomically normal small bowel; eight of these were over 80 years old, and 10 had vitamin B12 deficiency of whom five had confirmed pernicious anaemia. Enterobacterial overgrowth was a feature of all diagnostic groups except pancreatic and coeliac disease. Vitamin B12 deficiency may be an effect of malabsorption, but can also be a cause through impairment of enterocyte function. The association of pernicious anaemia and B12 deficiency with otherwise unexplained malabsorption and bacterial overgrowth suggests that gastric atrophy is a major causal factor in this syndrome, combined in some cases with a 'vicious circle' of B12 malabsorption and deficiency.