BACKGROUND AND OBJECTIVES: Exposure to negative social interactions (such as rudeness) has robust adverse implications on medical team performance. However, little is known regarding the effects of positive social interactions. We hypothesized that expressions of gratitude, a prototype of positive social interaction, would enhance medical teams' effectiveness. Our objective was to study the performance of NICU teams after exposure to expressions of gratitude from alternative sources. METHODS: Forty-three NICU teams (comprising 2 physicians and 2 nurses) participated in training workshops of acute care simulations. Teams were randomly assigned to 1 of 4 conditions: (1) maternal gratitude (in which the mother of a preterm infant expressed gratitude to NICU teams, such as the one that treated her child), (2) expert gratitude (in which a physician expert expressed gratitude to teams for participating in the training), (3) combined maternal and expert gratitude, or (4) control (same agents communicated neutral statements). The simulations were evaluated (5-point Likert scale: 1 = failed and 5 = excellent) by independent judges (blind to team exposure) using structured questionnaires. RESULTS: Maternal gratitude positively affected teams' performances (3.9 6 0.9 vs 3.6 6 1.0; P = .04), with most of this effect explained by the positive impact of gratitude on team information sharing (4.3 6 0.8 vs 4.0 6 0.8; P = .03). Forty percent of the variance in team information sharing was explained by maternal gratitude. Information sharing predicted team performance outcomes, explaining 33% of the variance in diagnostic performance and 41% of the variance in therapeutic performance. CONCLUSIONS: Patient-expressed gratitude significantly enhances medical team performance, with much of this effect explained by enhanced information sharing.