Fourteen adults in whom diabetes mellitus and coeliac disease coexist, are described. In no patient was coeliac disease diagnosed (biopsy proven) before the age of 28 years. Diabetes was recognized before coeliac disease in all except one. Diabetic control was very unstable and hypoglycaemia particularly troublesome before treatment with a gluten free diet. Following gluten restriction, insulin requirement increased in six patients, and diabetic control became more stable. Diarrhoea due to coeliac disease in a patient with coexisting diabetes, may be mistakenly diagnosed as 'diabetic diarrhoea'. However, certain clinical and laboratory features should arouse suspicion that the diarrhoea is not of diabetic origin. These included a history of gastrointestinal symptoms preceding the diagnosis of diabetes, the occurrence of repeated hypoglycaemia, absence of neuropathy, anaemia, low serum folate, low serum albumin and a malabsorption pattern on small bowel radiography. A definitive diagnosis of coeliac disease can be made only jejunal biopsy. The opportunity to diagnose coeliac disease in adult diabetics will usually fall to the diabetologist and wider use of jejunal biopsy in diabetics with chronic or recurrent diarrhoea is suggested.