Sixty-four diabetic patients, 35 with diarrhea, 15 with constipation and 14 without stool problems, and forty healthy subjects, were subjected to rectosigmoidoscopy. During rectosigmoidoscopy, rectal biopsy specimens for histological and histochemical analysis were obtained. Histological findings of nonspecific colitis in 25 out of 64 diabetic patients were uniformly distributed among the three groups (p = 0.959). However, the finding was slightly more common in diabetic patients than in controls (eight out of 40 control subjects, p = 0.043). A positive PAS reaction was observed in 30 out of 64 diabetic patients and was also uniformly distributed among the three groups (p = 0.508), but was significantly more common among diabetic patients than controls (three out of 40, p < 0.001). A positive reaction to cholesterol was found in 46 out of 64 diabetic patients, also uniformly distributed among the three groups (p = 0.773). It was significantly more common in diabetic patients than in controls (nine out of 40, p < 0.001). Reactions of the rectal mucosa histological specimens to glycogen and triglycerides were negative, both in diabetic patients and in controls. In conclusion, it appears that stool problems among our diabetic patients were not related to the positivity of PAS or to the positive cholesterol reaction in the rectal mucosa histological specimens. Since positive findings of both reactions were more common in specimens taken from diabetic patients than in controls, positive reactions might be related to metabolic disturbances in diabetic patients.