OBJECTIVE To analyse factors associated with the lack of condom use among young people at last sexual intercourse with a steady or casual partner. METHODS A cross-sectional study involving 1170 household interviews and designed to build a representative sample of the population of young Brazilian residents aged 15-24 years (2003). METHODS In the multivariate analysis of data, non-conditional logistic regression modelling was applied to assess the determinants of condom use at last sexual intercourse among young people with steady or casual partners. RESULTS The overall level of condom use at last sexual intercourse was high (60%), although it was significantly more common in casual sexual partnership. Cohabitation was associated with a lack of condom use in both casual and steady partner encounters. In addition, being female, having less schooling, having no work history, and per capita family income above the minimum wage were factors related to not using condoms in the group of young people who had their last sexual encounter with steady partners. Among young people with casual partners, such factors included a positive history of alcohol use, first sex at 9-16 years of age, inadequate knowledge of AIDS treatability and bereavement related to violence. CONCLUSIONS This study confirmed that the determinants of condom use among youth during last sexual intercourse vary according to whether the partner was casual or steady. Prevention campaigns should develop specific messages for each of these contexts.