In the present study, we describe the innovative use of the National Board of Medical Examiners (NBME) Comprehensive Basic Science Examination (CBSE) as a progress test during the preclerkship medical curriculum. The main aim of this study was to provide external validation of internally developed multiple-choice assessments in a new medical school. The CBSE is a practice exam for the United States Medical Licensing Examination (USMLE) Step 1 and is purchased directly from the NBME. We administered the CBSE five times during the first 2 yr of medical school. Student scores were compared with scores on newly created internal summative exams and to the USMLE Step 1. Significant correlations were observed between almost all our internal exams and CBSE scores over time as well as with USMLE Step 1 scores. The strength of correlations of internal exams to the CBSE and USMLE Step 1 broadly increased over time during the curriculum. Student scores on courses that have strong emphasis on physiology and pathophysiology correlated particularly well with USMLE Step 1 scores. Student progress, as measured by the CBSE, was found to be linear across time, and test performance fell behind the anticipated level by the end of the formal curriculum. These findings are discussed with respect to student learning behaviors. In conclusion, the CBSE was found to have good utility as a progress test and provided external validation of our new internally developed multiple-choice assessments. The data also provide performance benchmarks both for our future students to formatively assess their own progress and for other medical schools to compare learning progression patterns in different curricular models.