Objectives: Health risk-taking behaviors such as smoking, alcohol consumption, and sexual intercourse among adolescents are significant public health problems. Recent data indicate that smoking, alcohol consumption, and sexual intercourse in adolescents have been increasing steadily in South Korea. Few studies of smoking, alcohol consumption, and sexual intercourse for adolescents have been conducted simultaneously. This study is to examine relationships with socio-demographic variables, current smoking, dosage of alcohol consumption, sexual-related behaviors and sexual intercourse among adolescents in Seoul city, Korea. Methods: The population of this study was junior and senior high students in Seoul, Korea. Eight junior high schools and eight senior high schools were randomly selected from Seoul City. Two thousand nine hundred and ninety-six responses from the sixteen schools were analyzed (fifty-nine were excluded). Adolescents' self-reports on smoking, d alcohol behavior, sexual related behaviors and sexual intercourse were used. Results: The proportions of sexual intercourse were 2.6 % for middle school students and 7.2% for high school students. Boys were more likely to take part in sexual behaviors (use of pornographic materials, masturbation, kiss, and sexual intercourse). Tobacco use, alcohol consumption, use of pornographic materials, masturbation, kiss, and self-control were found to be associated with each other reciprocally. Results revealed that Tobacco use, alcohol consumption, use of pornographic materials, masturbation, kiss, and self-control predicted sexual intercourse. For male adolescents, tobacco use, alcohol consumption, and self-control predicted sexual intercourse. For female students, tobacco use, use of pornographic materials, masturbation, kiss, and self-control predicted sexual intercourse. Conclusion: This study suggests that adolescents are risk-taking behaviors simultaneously. To prevent adolescents health risk-taking behaviors we should develop practical educational principles that focus on delaying a young persons first use of cigarettes and alcohol. Health education should be considered to establish a compulsory curriculum as part of the school curriculum to offer health education systematically and comprehensively.