BACKGROUND: Interdisciplinary rounds (IDR) provide a means to assemble hospital team members and improve collaboration. Little is known about teamwork during IDR. OBJECTIVE: To evaluate and characterize teamwork during IDR. DESIGN: Cross-sectional observational study. SETTING: Six medical units which had implemented structured interdisciplinary rounds (SIDR). MEASUREMENTS: We adapted the Observational Teamwork Assessment for Surgery (OTAS) tool, a behaviorally anchored rating scale shown to be reliable and valid in surgical settings. OTAS provides scores ranging from 0 to 6 (0 = problematic behavior; 6 = exemplary behavior) across 5 domains (communication, coordination, cooperation/backup behavior, leadership, and monitoring/situational awareness) and for prespecified subteams. Two researchers conducted direct observations using the adapted OTAS tool. RESULTS: We conducted 7-8 independent observations for each unit (total = 44) and 20 joint observations. Inter-rater reliability was excellent at the unit level (Spearman's rho = 0.75), and good across domains (rho = 0.53-0.68) and subteams (rho = 0.53-0.76) with the exception of the physician subteam, for which it was poor (rho = 0.35). Though teamwork scores were generally high, we found differences across units, with a median (interquartile range [IQR]) 4.5 (3.9-4.9) for the lowest and 5.4 (5.3-5.5) for the highest performing unit (P < 0.01). Domain scores differed, with leadership receiving the lowest (median [IQR] = 5.0 [4.6-5.3]), and cooperation/backup behavior and monitoring/situational awareness receiving highest scores (median [IQR] = 5.4 [5.0-5.5] and 5.4 [5.0-5.7]). Differences across subteams were of borderline significance (P = 0.05). CONCLUSIONS: The adapted OTAS instrument demonstrated acceptable reliability for assessing teamwork during SIDR across units, domains, and most subteams. Variation in performance suggests a need to improve consistency of teamwork and emphasizes the importance of leadership.