Background: The proportion of adolescents experiencing unwanted pregnancy and abortion caused by the premature initiation of sexual intercourse is increasing at an alarming rate in Korea. This study aimed at developing a theoretical model for identifying individual and environmental risk factors affecting the initiation of sexual intercourse by adolescents. Methods: A descriptive survey was used to identify the predictors of sexual intercourse among Korean adolescents. Data were collected from students attending 10 senior high schools in 2 cities and 3 provinces in Korea. A total of 1012 students were chosen by convenience sampling, with 50 of them being excluded due to their failing to respond properly. Results: Logistic regression analysis of the data showed that gender, socialization traits, and disinhibition were the statistically significant individual factors, and that parental living arrangement, career track, school record, having a boy-/girlfriend, and sexual permissiveness with a boy-/girlfriend were the statistically significant environmental factors associated with sexual intercourse by adolescents. Conclusion: The theoretical model developed on the basis of the above findings should be useful in the development of effective counseling and preventive intervention programs provided to adolescents by school and community nurses. (Contains 3 tables and 1 figure.)