ABSTRACT: Patients with diabetes mellitus may have gastrointestinal complications of gastroparesis diabeticorum, bacterial overgrowth and/or diabetic diarrhea. Patients with diabetic diarrhea thought to be secondary to diabetic neuropathy usually have ten to 30 stools/day, with large volumes of more than 200 g/day, but weight loss is distinctly unusual. Gastroporesis diabeticorum may have weight loss associated with its gastric dilation and retention of food if vomiting is present. A woman with both gastroporesis diabeticorum, diabetic diarrhea, and, in addition, possible bacterial overgrowth who had a marked weight loss of 52 pounds without any vomiting is described. This patient, who had a normal fecal fat excretion and negative stools for bacteria, ova, and parasites, did respond to tetracycline each time her diarrhea reappeared after several diarrhea-free months in a 3-year follow-up. Metoclopramide did not help this patient and may have contributed to worsening her diarrhea. Thus, even if bacterial overgrowth cannot be documented by quantitative bacterial cultures of intestinal contents, a trial of antibiotics appears reasonable when other causes of diarrhea have been ruled out in diabetics. This treatment allowed the patient to regain weight she had lost.