Admissions and personnel decisions rely on stable predictor-criterion relationships. The authors studied the validity of Big Five personality factors and their facets for predicting academic performance in medical school across multiple years, investigating whether criterion-related validities change over time. In this longitudinal investigation, an entire European country's 1997 cohort of medical students was studied throughout their medical school career (Year 1, N = 627; Year 7, N = 306). Over time, extraversion, openness, and conscientiousness factor and facet scale scores showed increases in operational validity for predicting grade point averages. Although there may not be any advantages to being open and extraverted for early academic performance, these traits gain importance for later academic performance when applied practice increasingly plays a part in the curriculum. Conscientiousness, perhaps more than any other personality trait, appears to be an increasing asset for medical students: Operational validities of conscientiousness increased from .18 to .45. In assessing the utility of personality measures, relying on early criteria might underestimate the predictive value of personality variables. Implications for personality measures to predict work performance are discussed.