Medical simulation is associated with intense emotions which influence human behavior. We aim to investigate how prebriefing impacts on learners' emotions during a high-fidelity simulation (HFS) session. This is a prospective randomized controlled study. Participants were randomly allocated to receive a standardized prebriefing (SP group) versus not receiving it (NSP group). Debriefing following the «good judgment» approach, structured in reactions, understanding and summary phases, was used in both groups. In order to assess emotions, we used the circumplex model of affect applying the Affect Grid scale, which was performed prior to prebriefing, following case performance and following debriefing. Debriefing times were also assessed. A total of 128 physicians participate in the study (64 vs. 64). Following case performance, this HFS session was experienced with significantly more pleasant emotions compared to baseline, that were maintained during debriefing (p < 0.01) while alertness increased after case performance diminishing after debriefing (p < 0.01). There were no statistical significant differences between groups. In the NSP group, total debriefing (p = 0.003) and understanding phase (p = 0.002) times were significantly longer. This HFS session was experienced as pleasant with high alertness with no specific emotional impact attributable to prebriefing. Prebriefing leads to a freer flowing debriefing.