The association between poor school bonding and delinquency has only been partly addressed in earlier research. Using a longitudinal design, the objective of our study was to investigate possible bidirectional effects and sex differences between adolescents' experienced school bonding and self-rated delinquency over time. A total of 788 adolescents (353 boys and 435 girls) were investigated by questionnaire at age 14 and 16. Poor school attachment and commitment as well as poor teacher attachment were found to be stronger determinants of delinquency for males than for females. Delinquency predicted poor school commitment for both boys and girls, and poor school attachment for girls, thus indicating bidirectional effects over time and sex differences in some of the bidirectional effects. The study concludes that the delinquency propensity of adolescent boys may be affected by all school bonding dimensions, whereas for girls it is their relationship with their teachers that is of significance. (Contains 2 tables and 2 figures.)