Objective To study the relationship between the irritable bowel syndrome and bacterial overgrowth in the intestines.Methods Forty-nine cases of irritable bowel syndrome presenting as diarrhea or constipation were tested by the lactose-functose hydrogen breath test to determine the oral-cecal passage time and positive rate was noted.Of the cases with positive tests ten were selected for treatment with prepulsid for one week,and the pre-and post-treatment passage times were compared.Results In regard to passage time both diarrhea and constipation types of the irritable bowel syndrome showed prolongation compared with controls(P0 05),and improvement after treatment compared with pre-treatment,diarrhea was also improved by treatment.Conclusion Intestinal dysfunction can lead to bacterial overgrowth in the intestines,which can be a cause of the irritable bowel syndrome.