BACKGROUND: The optimal retraining intervals for Immediate Life Support (ILS) are unclear. This study aimed to explore the effects of different retraining intervals for simulation-based, short-duration ILS courses. METHODS: In this randomized controlled study, junior residents and nurses were recruited and assigned to three groups. After receiving initial simulation-based ILS training, the groups underwent retraining at different intervals: 3 months, 6 months, and 1 year. Each one-hour retraining session included an 8-min in-situ resuscitation simulation on a high-fidelity manikin, followed by debriefing. One year after the initial training, all participants completed a paper-based test and self-efficacy questionnaires on teamwork performance, in addition to a resuscitation simulation. Blinded evaluators assessed performance by reviewing simulation videos using validated checklists. RESULTS: Eighty-two out of 89 participants completed the study. They had similar characteristics, including age and years of work experience. The 6-month group had fewer resuscitation experiences in the past year. One year after the initial training, there were significant differences in the median skill performance scores across the groups (3-month vs. 6-month vs. 1-year: 31 vs. 28 vs. 23.5, p <
0.01). The 3-month group outperformed the 6-month group (p = 0.04), and the 6-month group outperformed the 1-year group (p = 0.01). The 3-month group also had significantly higher knowledge scores and performed best in self-evaluated teamwork performance. CONCLUSION: Our study shows that a 3-month retraining interval achieved the greatest effect for healthcare professionals with limited resuscitation experience in simulation-based, short-duration ILS retraining courses.