Using the adverse events monitoring system of Japan, we observed diarrhea cases in approximately 10% of patients who received oral poliovirus vaccine (OPV). This study was conducted to investigate whether diarrhea among children aged 0 to 1 is caused by OPV or by other factors such contact at the doctor's office and/or with others outside the home. We conducted a survey of the health of children after regular health check-ups and after the administration of the OPV. The data from the health check-ups were used as a control for the OPV case group. We compared the first-OPV dose vaccination group as well as the second-OPV dose vaccination group to the health check-up group. For cases of diarrhea, the odds ratio of the OPV group to the health check-up group was 1.776. Our findings strongly suggest that post-OPV cases of mild diarrhea are closely related to the administration of the OPV.