Adolescent health problems are often manifestations of problems having a social origin. More than half of all teenagers are sexually active, placing themselves at risk for unwanted pregnancies and sexually transmitted diseases. There is a strong positive association between early sexual experimentation and substance use of any kind. To meet adolescent health care needs, there were 120 school-based clinics in 61 communities across the United States as of 1988. The number of school-based clinics has risen since the early 1980's in response to the health problems of adolescents and their difficulties in using the existing health care system. School-based clinics focus on preventive health care and offer comprehensive, accessible, and appropriate services for teenagers. They are situated where many adolescents spend a large proportion of their time, and almost all clinics are operated by entities outside the school system who work cooperatively with the school administration. These entities include hospitals and medical schools, community clinics, public health departments, and private nonprofit organizations. The legal framework for providing health care to adolescents involves minor consent to treatment, informed consent, emergency care, the treatment of sexually transmitted and contagious diseases, family planning, pregnancy, drug and alcohol use, mental health counseling, and services for AIDS and HIV infection. Confidentiality and liability are also significant legal issues in operating school-based health clinics. 6 tables.