In this paper we study whether parental knowledge of adolescents' activities varies according to socio-demographic variables, and we analyze the possible association between parental knowledge patterns and certain risk behaviors among adolescents. A cross-sectional study was performed with representative samples of high-school students in Peru and El Salvador. A questionnaire assessed risk behaviors, as well as possible determinants, including parental knowledge. The questionnaire was answered by 6208 adolescents. We observed that the greater the degree of knowledge, the lower the frequency of risk behaviors among youth. The degree of knowledge was inversely associated with children's age, and we observed that being female was associated with a greater degree of parental knowledge. The study shows that parents' supervision criteria might be influenced by gender stereotypes, which would have a harmful effect on young males, as the lower degree of knowledge puts them at higher odds of risk behaviors.