Background The causes and mechanisms of chronic diarrhea are complex.This study aimed to explore the relationship between chronic diarrhea with normal colonoscopy findings and terminal ileum lesions.Methods All cases were collected from January 2009 to June 2010.The 40 patients in the patient group had chronic diarrhea with normal colonoscopy findings.Those who had hyperthyroidism,diabetes,chronic pancreatitis,cirrhosis,atrophic gastritis,short bowel syndrome and connective tissue diseases had been excluded.The control group contained 40 healthy individuals without diarrhea.Endoscopy of the terminal ileum was applied in both groups,with the endoscope inserted into terminal ileum for more than 20 cm.The patients diagnosed of chronic diarrhea with terminal ileum lesions were treated with metronidazole and probiotics for 10-14 days.Results Before treatment there were significant differences in endoscopy findings of the terminal ileum between the two groups (P ＜0.05).In the patient group,endoscopy showed congestion,edema,erosion and ulcers in 29 cases,hyperplasia and enlargement of lymphoid follicles in 10 cases with a maximal diameter of 7-8 mm,and 1 case showed normal endoscopy results.After treatment,35 patients recovered from diarrhea,and terminal ileum lesions disappeared in 30 cases as determined by endoscopy.In the control group,endoscopy showed scattered hyperplasia of lymphoid follicles in 5 cases,and the follicles were small with the maximal diameter being 3 mm.There was no hyperemia,edema,erosion or ulcers.Conclusions Chronic diarrhea patients with normal colonoscopy findings may have lesions in the terminal ileum that can be detected by endoscopy; including hyperemia,erosion,ulcers and lymphoid follicle hyperplasia.Therapeutic effect is good with metronidazole and probiotics.