Six patients with severe watery diarrhoea were found to have microscopic total colitis. None had any abnormality detectable by conventional tests used to diagnose inflammatory bowel disease-namely, barium radiology and endoscopy. The diagnosis could only be made by microscopic examination of biopsy specimens from the apparently normal colon. Anaemia, raised erythrocyte sedimentation rate, hypokalaemia, and hypoalbuminaemia were common findings. Small-bowel function was normal in all, though three patients had jejunal lesions of uncertain relevance but seemingly unrelated to the diarrhoea. The five patients given anti-inflammatory drugs showed a satisfactory response with improvement of the diarrhoea and colonic inflammation and return to normal of the abnormal laboratory findings. Microscopic colitis is responsible for a proportion of cases of intractable diarrhoea of obscure origin and rectal and colonic biopsies sould be undertaken in such cases.