Background: Working in ad hoc teams in a health care environment is frequent but a challenging and complex undertaking. One way for teams to refine their teamwork could be through post-resuscitation reflection and debriefing. However, this would require that teams have insight into the quality of their teamwork. This study investigates (1) the accuracy of the self-monitoring of ad hoc resuscitation teams and their leaders relative to external observations of their teamwork and (2) the relationship of team self-monitoring and external observations to objective performance measures. Methods: We conducted a quantitative observational study of real-world ad hoc interprofessional teams responding to a simulated cardiac arrest in an emergency room. Teams consisting of residents, consultants, and nurses were confronted with an unexpected, simulated, standardized cardiac arrest situation. Their teamwork was videotaped to allow for subsequent external evaluation on the team emergency assessment measure (TEAM) checklist. In addition, objective performance measures such as time to defibrillation were collected. All participants completed a demographic questionnaire prior to the simulation and a questionnaire tapping their perceptions of teamwork directly after it. Results: 22 teams consisting of 115 health care professionals showed highly variable performance. All performance measures intercorrelated significantly, with the exception of team leaders' evaluations of teamwork, which were not related to any other measures. Neither team size nor cumulative experience were correlated with any measures, but teams led by younger leaders performed better than those led by older ones. Conclusion: Team members seem to have better insight into their team's teamwork than team leaders. As a practical consequence, the decision to debrief and the debriefing itself after a resuscitation should be informed by team members, not just leaders.