Idiopathic diarrhea is a common complication of diabetes mellitus. It occurs frequently, but not exclusively, in patients with poorly controlled insulin-dependent diabetes who also have evidence of diabetic peripheral and autonomic neuropathy. Associated steatorrhea is common and does not necessarily imply a concomitant gastrointestinal disease. The diarrhea is often intermittent; it may alternate with periods of normal bowel movements, or with constipation. It is typically painless, and occurs during the day as well as at night and may be associated with fecal incontinence. Multiple pathogenic mechanisms have been implicated, autonomic neuropathy, bacterial overgrowth, and pancreatic exocrine insufficiency being the most important underlying aberrations. However, diabetic diarrhea does not have a uniform and unequivocal pathogenesis. The diagnosis depends on a judicious clinical assessment accompanied by a stepwise laboratory evaluation, which allows the differentiation idiopathic diabetic diarrhea from the many other causes of diarrhea that can occur in diabetic and nondiabetic patients. The management can be difficult but many therapies, including antibiotics to eradicate bacterial overgrowth, as well as antidiarrheal agents, oral and topical clonidine, and somatostatin analogues may be effective in controlling diabetic diarrhea.