For two decades, school-based clinics have been providing basic health care to medically underserved teenagers and addressing the increasingly complex health and social problems facing young people, particularly unintended pregnancy. Today there are 150 school-based clinics operating in most major cities and many rural areas. In 1984, the Center for Population Options launched a major project to evaluate a diverse group of clinics located in different parts of the country. The project sought to asses student use of clinic services and the clinics' effects on use of medical services generally and on student absenteeism, illegal drug use, alcohol consumption, cigarette smoking, and unprotected sexual intercourse. The study also sought to examine clinics' potential to meet desired objectives and identify helpful strategies. The clinics selected for evaluation were in the following cities: Gary, Indiana; Muskegon, Michigan; Jackson, Mississippi; Dallas, Texas; Quincy, Florida; and San Francisco, California. All six clinics served low-income populations, provided primary health care, and were open daily during school hours. Evaluation data were drawn from four sources: (1) staff and student interviews; (2) clinic records; (3) a student health survey; and (4) longitudinal birthrate data. Very large percentages of students used clinics in five of the schools studied. Study results suggest that school-based clinics can reduce students' consumption of alcohol and tobacco. The clinics did not encourage students to be sexually active, even when dispensing or prescribing contraceptives. According to survey data, the clinics had no effect on pregnancy rates. Recommendations to improve clinics' effectiveness are provided. Included are two appendices containing a reproductive health and pregnancy prevention inventory and the student health survey. (33 references) (MLH)