Athletes and nonathletes use anabolic-androgenic steroids (AAS) to improve their strength and enhance their appearance. Few studies have been undertaken in the 1990s to assess the use of AAS in adolescents, following changes in legislation. This study was conducted to determine the prevalence of AAS use by high school students in Denver, Colorado, an area with high sports participation. A confidential questionnaire was completed by 6,930 students (response rate 96.6%) at 10 Denver high schools. The prevalence of AAS use was 2.7% (4.0% for boys and 1.3% for girls). Prevalence was slightly higher in sport participants than nonparticipants. The mean age of starting AAS was 14 years (range 8-17 years). This study is the first documented detailed assessment of high school students' knowledge of the risks of AAS. It shows knowledge deficits regarding potential side effects. Users of AAS were less likely than nonusers to acknowledge the risks of AAS. Only 18% of students claim to have been informed about AAS by physicians. The results suggest that health care providers should provide more information to adolescents about AAS. If educational efforts are found effective, teaching efforts regarding AAS should start in junior high school or sooner and continue through high school.