This study sought to assess the professionalism of 2nd- year medical students in the context of team-based learning. Background: Professionalism is an important attribute for physicians and a core competency throughout medical education. Preclinical training often focuses on individual knowledge acquisition with students working only indirectly with faculty assessors. As such, the assessment of professionalism in preclinical training continues to present challenges. We propose a novel approach to preclinical assessment of medical student year medical students completed self- and peer assessments of professionalism in two courses (Pediatrics and Renal/Genitourinary Diseases) following a series of team-based learning exercises. Assessments were composed of nearly identical 9-point rating scales. Correlational analysis and linear regression were used to examine the associations between self- and peer assessments and the effects of predictor variables. Four subgroups were formed based on deviation from the median ratings, and logistic regression was used to assess stability of subgroup membership over time. A missing data analysis was conducted to examine differences between average peer- assessment scores as a function of selective nonparticipation. Results: There was a significant positive correlation (r = .62, p < .0001) between self-assessments completed alone and those completed at the time of peer assessment. There was also a significant positive correlation between average peer-assessment and self-assessment alone (r = .19, p < .0002) and self-assessment at the time of peer assessment (r = .27, p < .0001). Logistic regression revealed that subgroup membership was stable across measurement at two time points (T1 and T2) for all groups, except for members of the high self-assessment/low peer assessment at T1, who were significantly more likely to move to a new group at T2, χ2(3, N = 129) = 7.80, p < .05. Linear regression revealed that self-assessment alone and course were significant predictors of self-assessment at the time of peer assessment (Fself_alone = 144.74, p <.01 and Fcourse = 4.70, p < .05), whereas average peer rating, stage (T1, T2) and academic year (13-14, 14-15) were not. Linear regression also revealed that students who completed both self-assessments had significantly higher average peer assessment ratings (average peer rating in students with both self-assessments = 8.42, no self-assessments = 8.10, self_at_peer = 8.37, self_alone = 8.28) compared to students who completed one or no self-assessments (F = 5.34, p <.01). Conclusions: When used as a professionalism assessment within team-based learning, stand-alone and simultaneous peer and self- assessments are highly correlated within individuals across different courses. However, although self-assessment alone is a significant predictor of self-assessment made at the time of assessing one’s peers, average peer assessment does not predict self-assessment. To explore this lack of predictive power, we classified students into four subgroups based on relative deviation from median peer and self-assessment scores. Group membership was found to be stable for all groups except for those initially sorted into the high self-assessment/low peer assessment subgroup. Members of this subgroup tended to move into the low self- assessment/low peer assessment group at T2, suggesting they became more accurate at self-assessing over time. A small group of individuals remained in the group that consistently rated themselves highly while their peers rated them poorly. Future studies will track these students to see if similar deviations from accurate professional self-assessment persist into the clinical years. In addition, given that students who fail to perform self-assessments had significantly lower peer assessment scores than their counterparts who completed self-assessments in this study, these students may also be at risk for similar professionalism concerns in the clinical years; follow-up studies will examine this possibility.