PURPOSE: In this study, doctors were surveyed with a questionnaire to determine whether they performed simultaneous vaccination and whether there were any concerns about safety or anxiety. The purpose of this study was to determine any problems associated with doctors readily performing simultaneous vaccination. METHODS: A trained surveyor visited 241 doctors from every institution registered with the National Immunization Program (NIP) located within six districts (gu) in the City of Busan (Dongnae-gu, Geumjeong-gu, Yeonje-gu, Suyeong-gu, Busanjin-gu, Haeundae-gu); a total of 155 (64%) valid responses were obtained. RESULTS: Of the 155 respondents, 144 (93%) were already performing simultaneous immunizations and 141 (91%) had a positive view of the practice. However, among the 144 doctors performing simultaneous immunizations, 67 (47%) were not confident about its safety; side effects were seen after simultaneous immunization by 86 doctors, 35 (41%) of whom believed that the frequency or possibility of side effects in simultaneous immunizations was higher than that in sequential immunizations. CONCLUSIONS: The use of simultaneous immunization is expanding quickly. However, among the doctors performing simultaneous immunizations, a high percentage had concerns over its unproven safety and potential side effects, indicating the need for academic societies or government institutions to present evidence to address such concerns.