OBJECTIVE To identify factors associated with U.S. medical school matriculants' postbaccalaureate premedical program participation and to determine whether participation was associated with plans at medical school graduation to practice in underserved areas. METHODS Deidentified, individualized records of Association of American Medical Colleges (AAMC) Matriculating Student Questionnaire responses for the 1996- 2000 cohort of U.S. medical school matriculants were analyzed using multivariable logistic regression for associations with postbaccalaureate premedical program participation. Postbaccalaureate premedical program participation was analyzed for associations with plans at medical school graduation to practice in underserved areas. Adjusted odds ratios (ORs) significant at P < .05 are reported for independent predictors of postbaccalaureate program participation among matriculants and of graduates' plans to practice in underserved areas. RESULTS The sample of 57,276 matriculants included 3,561 (6.2%) academic record enhancer (ARE), 3,931 (6.9%) career changer (CC), and 1,354 (2.4%) career changer/academic record enhancer program participants (ARE/CC). Matriculants who participated in summer academic enrichment programs (OR = 1.35), had premedical debt (OR = 1.25), and were underrepresented minorities (OR = 1.21) were more likely to report ARE participation. Women (OR = 1.46) were more likely to report CC participation. Compared with nonparticipants, ARE, CC, and ARE/CC participants were each more likely to plan, at medical school graduation, to practice in underserved areas (OR = 1.14, 1.48, and 1.47, respectively). CONCLUSIONS Among medical school matriculants, postbaccalaureate premedical program participants were demographically diverse and, at medical school graduation, were more likely than nonparticipants to plan to practice in underserved areas.