The objective of this study was to assess dental students' attitudes toward underserved populations across their four years of dental school. Students at the Herman Ostrow School of Dentistry of the University of Southern California were invited to take part in the study. Participating students completed a questionnaire on their attitudes toward the underserved at three time points: 1) during orientation week; 2) at the end of their second year after taking part in some community dental programs; and 3) at the end of their fourth year after they had completed all their mandatory and volunteer rotations in community dental programs. Students' attitudes were measured in four categories: societal expectations, dentist/student responsibility, personal efficacy, and access to care. First-year students scored 85 out of a maximum of 115 on the questionnaire. Female students scored higher than male students (P=0.006). Age, debt, and past history of volunteer work were not related to first-year students' total attitude scores; however, students with a history of volunteer experience scored higher on the dentist/student responsibility category (P=0.04). Students' attitude scores declined across the four years of dental school (P=0.001). The same patterns were evident for all categories except societal expectations. The decline was not related to age, gender, debt, or volunteer work experience. Follow-up studies are needed to help explain the factors that may be related to this decline.