To assess risk sexual behaviour in adolescents and its relationship with family characteristics. In a representative and random sample of 909 teenagers, their sex life, structure, satisfaction and family dynamics were evaluated. It was used chi-squared test, in order to compare the frequency of family negative characteristics between the groups of adolescents with and without risky sexual behaviors. The early onset of sexual life was more frequent in adolescents with dysfunctional expression of affection in their families, and single-parent or nuclear family structure. The lack of condom use was associated with a lack of affection, and a poor comunication within the family. Having more than three partners was related to the expression of affection and the degree of satisfaction of the teenager with his family. Unplanned pregnancies and sex transmission diseases were most frequent in adolescents belonging to extended families with a poor expression of affection. The level of communication, the monitoring of conduct, the warmth and proximity play a very important role as protectors of sexual risk behaviour in teens.