Thirteen diabetic patients with chronic diarrhea have been studied clinically and subjected to jejunal biopsy. Eleven conformed to the accepted clinical picture of “diabetic diarrhea,” all showing evidence of gross neuropathy and a normal jejunal mucosa was found in every case. The other two were found to have a flat jejunal biopsy; in one celiac disease had been diagnosed at the age of two, but in the other there was no remote history of gastrointestinal disorder. The diagnosis of “diabetic diarrhea” is one of exclusion and there is no single feature which can be regarded as diagnostic. The nature of the diarrhea is uncertain but the response in some patients to antibiotics suggests an abnormal colonization of bacteria in the intestine though this cannot be demonstrated. The association of diarrhea with diabetic neuropathy is strong.