METHODS A 45-year-old man with type I diabetes mellitus was admitted to hospital because of colicky abdominal pain and 5-6 watery stools daily. Upper gastrointestinal endoscopy showed nearly total atrophy of the villi in the duodenum and jejunum suggesting coeliac disease. However, gluten-free diet for 2 weeks brought no improvement. Another examination of the biopsy 6 weeks after the first endoscopy revealed extensive collagen deposition in the lamina propria of the small intestine, giving the diagnosis of collagenous sprue. METHODS Parenteral nutrition, lactulose, cisapride, cholestyramine, doxycycline, paromomycin, vancomycin and octreotide failed to affect the loss of fluid from the gut which 12 weeks after admission had increased to 221 daily. However, it was stopped after prednisolone was administered (100mg daily). 7 months after starting the steroid treatment the collagen layer had disappeared and the villous atrophy had partially regressed. Over the next 6 months the prednisolone dosage was decreased to 10 mg daily. Shortly thereafter a perimembranous glomerulonephritis occurred, with proteinuria (up to 60 g/d) and oedema. It regressed to 6 g/d when the steroid dose was increased and cyclosporin, 0.5 g/d, had been added. On maintenance dosage of cyclosporin the histological and clinical remission of the collagenous sprue has now lasted for over 2 years. CONCLUSIONS This case suggests that steroid administration is an effective treatment of collagenous sprue. The presence of diabetes and other immune-related diseases in this case also suggests that an immunological mechanism may play a causative role in collagenous sprue.