This study's interest relies on adolescents' social representations of unprotected sex, more precisely on the relationship between the attitude towards the preservative and the reason attribution for its non use. 1386 secondary school students took part in the study, in the Brazilian cities of Florianópolis, Itajaí and Balneário Camboriú. In order to verify reasons attributed by the students, we focused on the sample that had sexual experiences without using the condom during last year. Data was analyzed with software ALCESTE, which showed three different classes of explanations for the non use of the preservative: the moment of the intercourse (unpredictable and incontrollable), trust in the partner and the option of the contraceptive pill, instead of the preservatives, in avoiding pregnancy. The students' attitudes towards the preservative are less favourable among those who maintain sexual intercourse with known people. The results revealed two representations of AIDS: one of trust in the partner and another of the experience with sex and the preservative – the first one gives sense to the adolescents' experiences with known sexual partners and the second, with less known sexual partners.