This study examined the relationship between participation in a school-based hepatitis B immunization program and teacher attitudes toward school-based health care and student socioeconomic factors. A survey addressing teachers' attitudes was administered to all teachers participating in the program. Information regarding student participation in school lunch programs and scores on national standardized tests were collected. Of the 4,874 fifth-grade students targeted for the program, 3,483 (72%) consented to be vaccinated and 3,232 (93% of 3,483) received all three doses of vaccine. Socioeconomic factors were the most important predictors of student participation in this school-based immunization program. Participation was significantly lower among students in schools with a high proportion of students receiving free or reduced-price school lunch and with low test scores. The only teacher factor associated with student participation was whether the teacher had returned the questionnaire. Strategies to increase immunization coverage in school-based programs should target children of low socioeconomic status.