BACKGROUND This study aimed to evaluate, in outpatient clinics in Poland between January 2022 and July 2022, the effectiveness of the But-You-Are-Free (BYAF) social influence technique by healthcare professionals during interaction with 185 parents deciding about vaccinating (eg, Hexacima, Prevenor 13, Synflorix, Rotateq, Act Hib, Boostrix, Pentaxim, DTP, Imovax, Priorix, MMR, Tetracim, Adacel, Euvax B, Fuvax, FSME, Varilix, Nimenrix, Bexero vaccines) their babies. MATERIAL AND METHODS During an interaction with pediatricians or nurses, the parents were encouraged to vaccinate their babies. In experimental condition (111 interactions), the BYAF technique was employed, and the phrase "But you are free" was added at the end of the conversation. In the control condition (74 interactions), it was not employed. RESULTS In the experimental condition, 71 (64%) participants declared intention to vaccinate their children. In the control condition, it 61 (84%) participants declared intention to vaccinate (the difference in percentages was significant: P=0.006). The number of parents who actually vaccinated their babies in the experimental condition was 92 (83%) and in the control condition it was 70 (95%; P=0.018). CONCLUSIONS The findings showed that participants who were exposed to the BYAF technique declared lower intentions to vaccinate their babies, and vaccinated them less often than those in the control condition. This result critically highlights that this technique should not be employed in the medical settings of pediatric vaccination.