OBJECTIVE: To investigate the prevalence of several health behaviors among students of public and private schools in S. Paulo, Brazil. METHODS: An epidemiological survey about health behaviors among high school students was carried out in S. Paulo in 1998. Seventh to eleventh graders from ten public and seven private schools were interviewed. All students were asked to fill out a Portuguese version of the questionnaire used by the Centers for Disease Control and Prevention in their annual "Youth Health Risk Behavior Survey". This questionnaire includes questions on driving-related behavior, violence, substance abuse, sexual practices, dietary habits and body weight control. RESULTS: A significant proportion of the students, mainly in the range of 15--18 years old, reported engaging in health-risk behaviors. In public schools, the most prominent risk behaviors were: riding a motorcycle without helmets (reported by 70.4% of the students who were either passengers or the driver); not using condoms in the last sexual intercourse (34% among those sexually active); carrying guns (4.8% in the last year), and suicide attempts (8.6% in the last year). In private schools, substance abuse was the most prominent risk-behavior: 25% reported at least one episode of binge-drinking in the last 30 days, 20.2% sniffed solvents at least once in the last year and 22.2% smoked marijuana in the same period; 13.8% reported using tobacco on a regular basis. Not wearing helmets while riding a motorcycle was also very high, reported by 66.3%. Female students reported less risk-behaviors, except for suicide attempts and unhealthy weight control methods. CONCLUSIONS: The information gathered could contribute to the development of preventive programs at school level, which takes into consideration the students' risk behaviors.