Probable bacterial contamination of the small bowel without an anatomical sump occurred in 5 elderly patients (age 68--94). They presented with severe general deterioration in health rather than with obvious features of malabsorption. This syndrome has not been clearly described before. Bacterial deconjugation of bile salts was indentified by the 14C-glycocholic-acid breath test. Investigations for other causes of malabsorption, including small-bowel biopsy and pancreatic scan, revealed no abnormality. After prolonged ill-health despite an adequate diet in hospital, all 5 subjects made a striking recovery after treatment with antibiotics. This condition may be quite common, particularly in malnourished elderly people.