We report a case of a 62-year-old man who developed watery diarrhoea after starting treatment with cimetidine for dyspepsia. Macroscopically, sigmoidoscopy and colonoscopy were normal. Histology revealed features consistent with a diagnosis of collagenous colitis. The diarrhoea is responding to treatment with prednisolone and withdrawal of cimetidine. We conclude that the collagenous colitis may have been drug induced.