Importance Physicians can demonstrate mastery of the knowledge that supports continued clinical competence by passing a maintenance of certification examination (MOCEX). Performance depends on professional learning and development, which may be enhanced by informal routine interactions with colleagues. Some physicians, such as those in solo practice, may have less opportunity for peer interaction, thus negatively influencing their examination performance. Objective To determine the relationship among level of peer interaction, group and solo practice, and MOCEX performance. Design, Setting, and Participants Longitudinal cohort study of 568 surgeons taking the 2008 MOCEX. Survey responses reporting the level of physicians’ peer interactions and their practice type were related to MOCEX scores, controlling for initial qualifying examination scores, practice type, and personal characteristics. Exposures Solo practice and amount of peer interaction. Main Outcomes and Measures Scores on the MOCEX and pass-fail status. Results Of the 568 surgeons in the study sample, 557 (98.1%) passed the examination. Higher levels of peer interaction were associated with a higher score (β = 0.91 [95% CI, 0.31-1.52]) and higher likelihood of passing the examination (odds ratio, 2.58 [1.08-6.16]). Physicians in solo (vs group) practice had fewer peer interactions (β = −0.49 [95% CI, −0.64 to −0.33), received lower scores (β = −1.82 [−2.94 to −0.82]), and were less likely to pass the examination (odds ratio, 0.22 [0.06-0.77]). Level of peer interaction moderated the relationship between solo practice and MOCEX score; solo practitioners with high levels of peer interaction achieved an MOCEX performance on a par with that of group practitioners. Conclusions and Relevance Physicians in solo practice had poorer MOCEX performance. However, solo practitioners who reported high levels of peer interaction performed as well as those in group practice. Peer interaction is important for professional learning and quality of care.