Most medical schools now include some component of professionalism in their curriculum, ranging from "white coat" ceremonies to didactic and small-group, case-based discussions. Often this format does not provide a context for the course content nor does it necessarily make the curricular themes relevant to population groups and communities most vulnerable to the inequities and injustices present in health care. The authors describe a community-based professionalism curriculum for preclinical and clinical year medical students and report evaluation data from three years (2001-2003) of this national demonstration project. The curriculum emphasized four themes: service, community, advocacy, and ethical behavior and was based on a service-learning pedagogy applied within community-based organizations. As part of the program evaluation, 95 students from 33 medical schools between the years 2001 and 2003 (response rate: 84.8%) completed an anonymous questionnaire. When asked what did they learn about professionalism that they did not learn (or expect to learn) in their medical school curriculum, the most common themes were (1) factors and influences affecting professional behavior, with many specifically citing pharmaceutical companies and insurance carriers (46.3%); (2) the role and importance of physician advocacy on behalf of their patients (37.9%); and (3) issues specific to the needs of vulnerable and disadvantaged populations (20.0%). This project demonstrates that community-based experiences can provide unique and relevant learning in a professionalism curriculum that can complement existing medical-school-based efforts.