Despite the general recognition that the sexual practices of adolescent boys place them at high risk of acquiring sexually transmitted infections (STIs), including HIV, and of causing unwanted pregnancies, advances in mapping their sexual behaviors have been slow. This study uses data recently collected from low-income areas of the city of Recife, Brazil, to study boys' age at first sexual intercourse and factors that hinder their use of condoms. These boys become sexually active at early ages, and despite their general awareness of HIV, they rarely use condoms, especially at ages younger than 15. Sustained family involvement in guiding boys is associated with later first intercourse and an increased use of condoms. Boys who describe themselves as shy with girls have later first intercourse, although the probability of their using condoms does not differ from that of other boys of their age. Higher socioeconomic status leads to earlier sexual activity for boys (in contrast with girls), but also to a greater likelihood of using condoms during first intercourse.