Summary. The purpose of this paper is to summarise a five-year longitudinal study of the effects of the transition from single-sex high schools to co-educational high schools. During this period two single-sex high schools serving the same geographical area formed two co-educational high schools. The results of the present investigation are presented in three parts: (1) the impact of the transition from the perspective of teachers and staff who taught at the schools before, during, and after the transition; (2) the performance of Year 10 students on externally moderated examinations in English and mathematics before, during, and after the transition, and (3) student responses to a multidimensional self-concept instrument before, during, and after the transition. In each part the differential effects of the transition on boys and on girls are examined. The findings suggest that the transition benefited both boys and girls in terms of multiple dimensions of self-concept and that these benefits were not at the expense of academic achievement for either boys or girls.