Professional Identity Formation (PIF), the process of internalizing a profession's core values and beliefs, is an explicit goal of medical education. The Professional Identity Essay (PIE), a developmental measure of the extent to which individuals have a complex and self-defined understanding of their professional role, is a tool to both study and scaffold PIF. PIE staging has internal reliability and response process validity and correlates with a validated measure of moral reasoning. In this study, we investigate whether PIF, as measured by PIE, changes during preclerkship training.Medical students in the class of 2019 completed the PIE during orientation to medical school (PIE#1) and 15 months later, during orientation to clerkships (PIE#2), to the same prompts. These written responses are PIF-staged by an expert rater.On average, PIF scores reveal that 46% of the group remained at the same stage as they were on entry to medical school, 42% scored at a higher stage of PIF, and 15% of students scored at a lower stage of PIF after pre-clerkship training.This result suggests that medical students are heterogeneous with respect to the development of their medical PIF early in medical school training.