Many medical schools assert that a racially and ethnically diverse student body is an important element in educating physicians to meet the needs of a diverse society. However, there is limited evidence addressing the educational effects of student body racial diversity. To determine whether student body racial and ethnic diversity is associated with diversity-related outcomes among US medical students. A Web-based survey (Graduation Questionnaire) administered by the Association of American Medical Colleges of 20,112 graduating medical students (64% of all graduating students in 2003 and 2004) from 118 allopathic medical schools in the United States. Historically black and Puerto Rican medical schools were excluded. Students' self-rated preparedness to care for patients from other racial and ethnic backgrounds, attitudes about equity and access to care, and intent to practice in an underserved area. White students within the highest quintile for student body racial and ethnic diversity, measured by the proportion of underrepresented minority (URM) students, were more likely to rate themselves as highly prepared to care for minority populations than those in the lowest diversity quintile (61.1% vs 53.9%, respectively; P < .001; adjusted odds ratio [OR], 1.33; 95% confidence interval [CI], 1.13-1.57). This association was strongest in schools in which students perceived a positive climate for interracial interaction. White students in the highest URM quintile were also more likely to have strong attitudes endorsing equitable access to care (54.8% vs 44.2%, respectively; P < .001; adjusted OR, 1.42; 95% CI, 1.15-1.74). For nonwhite students, after adjustment there were no significant associations between student body URM proportions and diversity-related outcomes. Student body URM proportions were not associated with white or nonwhite students' plans to practice in underserved communities, although URM students were substantially more likely than white or nonwhite/non-URM students to plan to serve the underserved (48.7% vs 18.8% vs 16.2%, respectively; P < .001). Student body racial and ethnic diversity within US medical schools is associated with outcomes consistent with the goal of preparing students to meet the needs of a diverse population.