Improving professional attitudes and behaviors requires critical self reflection. Research on reflection is necessary to understand professionalism among medical students. The aims of this prospective validation study at the Mayo Medical School and Cleveland Clinic Lerner College of Medicine were: (1) to develop and validate a new instrument for measuring reflection on professionalism, and (2) determine whether learner variables are associated with reflection on the gross anatomy experience. An instrument for assessing reflections on gross anatomy, which was comprised of 12 items structured on five-point scales, was developed. Factor analysis revealed a three-dimensional model including low reflection (four items), moderate reflection (five items), and high reflection (three items). Item mean scores ranged from 3.05 to 4.50. The overall mean for all 12 items was 3.91 (SD = 0.52). Internal consistency reliability (Cronbach's α) was satisfactory for individual factors and overall (Factor 1 α = 0.78; Factor 2 α = 0.69; Factor 3 α = 0.70; Overall α = 0.75). Simple linear regression analysis indicated that reflection scores were negatively associated with teamwork peer scores (P = 0.018). The authors report the first validated measurement of medical student reflection on professionalism in gross anatomy. Critical reflection is a recognized component of professionalism and may be important for behavior change. This instrument may be used in future research on professionalism among medical students. Anat Sci Educ 6: 232–238. © 2012 American Association of Anatomists.