The internal and external predictors of adolescent antisocial behaviour were investigated with 132 participants from youth homes and university psychology classes. Personality was the chosen internal predictor, and family environment and peer influence were the selected external predictors. The purpose of the current study was to examine these predictors, and their relative effects. Participants were given a five- part questionnaire to complete. The questionnaire included sections from the Young Adult Self- Report, Eysenck Personality Questionnaire, and the Family Environment Scale. Total antisocial behaviour was significantly predicted by age, psychoticism, neuroticism, social conformity, peer influence and achievement orientation in the participant's family. There were limited differences regarding sex, education level and ethnicity. It is concluded that personality contributes the most to adolescent antisocial behaviour. Implications for future research and treatment strategies are discussed.