This descriptive, cross-sectional study conducted in Ibadan, South-West Nigeria, explored sexual abstinence practices of in-school adolescents, factors influencing or obstructing abstinence, and knowledge of HIV and AIDS. A multi-stage sampling technique was used to identify 420 respondents who completed a semi-structured questionnaire and participated in focus groups. Eight focus groups were held, with nine participants on average. Qualitative data were transcribed and manually summarized, while the quantitative data was analyzed using the Statistical Package for the Social Sciences. Twelve percent of the sample had ever had sex and knowledge of HIV was high. Seventy-nine percent of males versus 98% of females abstained. Being female, not having a partner, not using alcohol and having a positive outlook on abstinence were predictors of abstinence (P 0.05). Abstinence was also associated with self-efficacy and negative perceptions of sexually-active peers (P 0.05). Focus group discussions suggest that involving parents, media, schools, faith-based institutions and non governmental organizations could improve abstinence levels. The multiple factors influencing abstinence must be taken into account when determining the effectiveness of abstinence interventions. Such programs are desperately needed.