To assess adolescent knowledge, perceptions, and attitudes about health care confidentiality. Anonymous self-report survey with 64 items addressing confidentiality issues in health care. Rural, suburban, and urban high schools in central Massachusetts. Students in ninth through 12th grades from three schools. A total of 1295 students (87%) completed the survey: 58% had health concerns that they wished to keep private from their parents, and 69% from friends and classmates; 25% reported that they would forgo health care in some situations if their parents might find out. There were differences in response by gender, race, and school. About one third were aware of a right to confidentiality for specific health issues. Of those with a regular source of care, 86% would go to their regular physician for a physical illness, while only 57% would go there for questions about pregnancy, the acquired immunodeficiency syndrome, or substance abuse that they wished to keep private. Sixty-eight percent had concerns about the privacy of a school health center. A majority of adolescents have concerns they wish to keep confidential and a striking percentage report they would not seek health services because of these concerns. Interventions to address confidentiality issues are thus crucial to effective adolescent health care.