Purpose: Determine whether the National Board of Medical Examiners (NBME) Subject Examination performance from six clerkships correlated with United States Medical Licensing Examination (USMLE) Steps 1 and 2 Clinical Knowledge (CK) Examination scores. Also, examine correlations between medical students' preclinical and clinical year mean cumulative grade point average (GPA), subject exam, and USMLE performance. Method: The sample consisted of 507 students from the 2008-2010 graduating classes from the authors' medical school. Pearson correlations followed by stepwise linear regressions were used to investigate variance in USMLE Steps 1 and 2 CK scores explained by subject exam scores and GPA. Results: Data from 484 (95.5%) students were included. USMLE Steps 1 and 2 CK scores had moderate-to-large positive correlations with all subject exam scores and with both GPA variables. Correlations between composite subject exam scores and USMLE Steps 1 and 2 CK exams were 0.69 and 0.77, respectively. Regression analysis demonstrated that subject exams and GPA accounted for substantial variance in Steps 1 and 2 CK exam scores (62% and 61%); when entered into the regression model first, primary care clerkship subject examination scores accounted for most of this variance. Conclusions: The moderate-to-large correlations between subject exam performance and USMLE scores provide reassurance that subject exam scores are associated with USMLE performance. Furthermore, the considerable variance in USMLE scores accounted for by primary care NBME scores may be due to primary care topics being reinforced through all clerkships and comprising a significant portion of the USMLE examinations, particularly Step 2 CK.