Collagenous colitis and ulcerative colitis are distinct disorders. A 67 year old woman with clinical and histological evidence of collagenous colitis had an abrupt symptomatic exacerbation while taking anti-inflammatory treatment with sulphasalazine and prednisone. Repeat colorectal endoscopy showed active mucosal inflammation and colonic biopsy specimens were consistent with active ulcerative colitis. After bowel rest, total parenteral nutrition, intensification of the anti-inflammatory regimen, and withdrawal of non-steroidal anti-inflammatory drugs (which she had taken continuously for osteoarthritis) diarrhoea abated. Colorectal biopsy specimens obtained when the patient's symptoms had improved showed inactive ulcerative colitis with no evidence of collagenous colitis. This may be the first case to be reported of the metachronous association of collagenous and ulcerative colitis.