PURPOSE: This study was designed to investigate the effect of intestinal subepithelial collagenous thickening on diabetic diarrhea because one of the seven patients diagnosed with collagenous colitis was diabetic. METHODS: Rectosigmoidoscopic rectal biopsies were taken from 50 diabetic patients (8 with and 42 without diarrhea), 20 nondiabetic patients with diarrhea, and 10 healthy patients. Histopathologic examinations and measurements of subepithelial collagen layers were performed on these biopsies. RESULTS: In diabetic patients who had diarrhea, the subepithelial collagen layer (SCL) was thicker than it was in diabetics without diarrhea (P < 0.05). In diabetic groups, the SCL was thicker than it was in both nondiabetics with diarrhea and those without diarrhea (P < 0.05). There was no statistical difference between nondiabetics with diarrhea and those without (P > 0.05). There was no correlation between collagen thickness, age, and diabetes duration (P > 0.05). CONCLUSION: It was concluded that there was a thickening of the colonic SCL in diabetic patients.