RESUMEN
Third-party informed consent for child and adolescent participation in research is a legal requirement that has been questioned by authors who argue that children over 10 are fully able to make decisions regarding this matter. The extent to which this requirement encumbers survey researches in this age range has not been fully reported. In order to understand the reasons for the inconsistent use of condoms among adolescent students in Rio de Janeiro, we designed a survey based on an anonymous self-reported questionnaire. Two informed consent terms were distributed: one for the adolescent and one for the legal representative signature. Participation was offered to all students aged 12-18 attending class at the day of the consent term distribution. Among 906 distributed legal represents consent terms, 734 (81%) were not returned. The final sample probably presented a bias of selection. Researchers must foresee third-party consent as a major encumbrance. There is a need for the definition of a range of interventions in which the adolescent might have the legal recognition of autonomy for decision about his/her voluntary participation.