Background: The risk of sexually transmitted diseases is high but opportunities of sexual education for adolescents are limited in Turkey. The aim of this study was to evaluate sexual attitudes and behaviors and to determine the predictors of sexual initiation among adolescents. Methods: A questionnaire designed by the researchers was administered to 861 senior year high school students in their classrooms. Results: The rates of masturbation and sexual intercourse and the median partner numbers among males were higher than females. The rates of having negative feelings after the first sex among females were higher than males. The rates of having sexual intercourse and the mean age at first sexual intercourse among males were similar to developed countries. However, the use of a condom at first intercourse was low. Lower academic performance and grade repetition experience among males and cigarette smoking among both gender groups were related to having sexual intercourse. Conclusions: This is one of the first studies about sexual risk-taking behaviors among high school students in Turkey. Differences found between gender groups regarding attitudes and behaviors reflect the social structure in Turkey. These findings may be helpful in producing effective solutions for improving education and preventive health care. (Contains 6 tables.)