The aim of the present study was to assess the influence of self-efficacy and past behaviour on young people's physical activity intentions using an augmented version of Ajzen's Theory of Planned Behaviour. We hypothesized that self-efficacy would exhibit discriminant validity with perceived behavioural control and explain unique variance in young people's intentions to participate in physical activity. We also expected that past physical activity behaviour would attenuate the influence of attitude, subjective norms, perceived behavioural control and self-efficacy on intention. The sample comprised 1,152 young people aged 13.5 +/- 0.6 years (mean +/- s) who completed inventories assessing their physical activity intentions, attitudes, subjective norms, perceived behavioural control, self-efficacy and past physical activity behaviour. A confirmatory factor analysis demonstrated that the constructs of the Theory of Planned Behaviour achieved discriminant validity. Furthermore, the measures of attitudes, subjective norms, perceived behavioural control and self-efficacy were significantly related to their respective belief-based measures, supporting the concurrent validity of the measures of the Theory of Planned Behaviour. A non-standard structural equation model demonstrated that attitude and self-efficacy were strong predictors of physical activity intention, but perceived behavioural control and subjective norms were not. Self-efficacy attenuated the influence of attitudes and perceived behavioural control on intention. Past behaviour predicted intention directly and indirectly through self-efficacy and attitude. The present findings demonstrate that young people with positive attitudes and high self-efficacy are more likely to form intentions to participate in physical activity. Furthermore, controlling for past physical activity behaviour revealed that the unique effects of self-efficacy and attitudes on young people's physical activity intentions were unaltered.