OBJECTIVE: To record variables of the sexual behaviour of high school students in Northern Greece and to investigate factors associated with the onset of sexual activity.METHODS: A questionnaire (Centers of Disease Control /Prevention) adapted to national characteristics, was distributed to students from a representative sample of 11 high schools (7 public, 2 private, 2 vocational). A total of 523 students (1st, 2nd, 3rd grade) completed the questionnaire. Frequencies of certain sexual behaviours were estimated and multivariable logistic regression models were used to evaluate correlates of the onset of sexual activity.RESULTS: Onset of sexual activity was reported by 48% of adolescents, half of whom stated this had occurred by the age of 15. At last sexual intercourse 76% had used a condom and 21% no contraception at all. Initiation of sexual activity was influenced by gender, age, type of school and smoking. Boys, students attending vocational high schools and smokers were significantly more likely to have been sexually active.CONCLUSIONS: Sexually-related behaviours among Greek high school students are quite frequent; they should be regularly registered. Sexual education programmes specifically designed for youths must be implemented.