Consultations with patients who bring drugs, especially on the high risk drug list, to a hospital is an important role of pharmacists. However, many incident reports occur though pharmacists generally make an effort to check such medications. In Japan, incidents are mostly reported just in terms of numbers but not in terms of the prevalence of a target group. We aim to reveal the prevalence of incidents related to medicine brought-in by patients undergoing surgery in National Hospital Organization (NHO) hospitals. For our study, we extracted patients undergoing surgery who were prescribed antidiabetic agents from the Medical data bank (MIA) in NHO. Chart reviews were performed on patients to evaluate the number of incidents in relation to brought-in medicine. The prevalence of incidents of interest was 4.4% (41/931, 95%CL : 3.2-5.9%). Pre-avoidable incidents represented 56.1% (23/41, p<0.0001). We found that pharmacists play a role in making incidents less severe.