OBJECTIVES: Using simulation-based mastery learning, residents can be trained to achieve a predefined performance standard in resuscitation. After mastery is achieved, performance degradation occurs over time. Prior investigations have shown performance retention of 12-14 months following intensive simulation-based mastery learning sessions. We sought to investigate the duration of mastery-level resuscitation performance retention after a single 1- to 2-hour simulation-based mastery learning session. DESIGN: Randomized, prospective trial. SETTING: Medical simulation laboratory. SUBJECTS: Convenience sample of 42 pediatric residents. INTERVENTIONS: Baseline resuscitation performance was determined on four standardized simulation scenarios. After determination of baseline performance, each resident repeated each scenario, as needed, until mastery-level performance was achieved. Residents were then randomized and retested 2, 4, or 6 months later. Statistical analysis on scores at baseline and retesting were used to determine performances changes from baseline and performance retention over time. MEASUREMENTS AND MAIN RESULTS: Forty-two residents participated in the study (12 in 2 mo group, 14 in 4 mo group, and 16 in 6 mo group). At baseline, postgraduate year-3 residents performed better than postgraduate year-1 residents (p = 0.003). Overall performance on each of the four scenarios improved at retesting. The percent of residents maintaining mastery-level performance showed a significant linear decline (p = 0.039), with a drop at each retesting interval; 92% retained mastery at 2 months, 71% at 4 months, and 56% at 6 months. There was no difference in retention between postgraduate year-1, postgraduate year-2, and postgraduate year-3 residents (p = 0.14). CONCLUSIONS: Residents displayed significant improvements in resuscitation performance after a single simulation-based mastery learning session, but performance declined over time, with less than 60% retaining mastery-level performance at 6 months. Our results suggest that relatively frequent refresher training is needed after a single simulation-based mastery learning session. Additional research is needed to determine the duration of performance retention following any specific simulation-based mastery learning intervention.