Early coital debut is a risk factor for HIV. In this paper we investigate the predictors of young adolescents' transition to first intercourse using a social cognition theoretical framework. The analyses reported here were based on a longitudinal study of 2360 students in the schools allocated to the control arm of a cluster-randomized controlled trial to investigate the effect of a school-based HIV prevention programme among Grade 8 students in Cape Town. Structural equation modelling was performed with Mplus version 3.11. Of the 1440 students who were virgins at baseline, 1144 remained virgins 15 months later and 296 (20.6%) reported having had their first sexual intercourse. Transition to first sexual intercourse was more likely among males than females, among older students and among students with a lower socio-economic status. Transition to first sexual intercourse was significantly associated with intentions to have sexual intercourse, poor self-efficacy to negotiate delayed sex and intimate partner violence. The model predicted 35% of the variance in intentions and 16% of the variance in transition. These findings indicate some of the factors that influence young adolescent's transition to first intercourse and that need to be addressed when designing effective interventions. (Contains 3 tables and 1 figure.)