Particularly in the 1960s and 1970s it was frequently argued that coeducational (Coed) high schools provided a more natural social environment to prepare adolescents for adulthood than did single sex (SS) schools. Based on the assumed accuracy of this belief, SS schools are becoming infrequent or even nonexistent in most western societies. This investigation was designed to provide information about the effects of the transition from SS to Coed high schools. This 5-year longitudinal study examined the effects of the transition from SS to Coed high schools when a boys' high school and girls' high school were reorganized into two Coed high schools, involving approximately 2,250 students sometime during the period from 1982 through 1985. Students were compared on self-concept and academic achievement before, during, and after the transition, and detailed perceptions were also collected from teachers who taught at the schools before, during, and after the transition. For both boys and girls there was a clear increase in multidimensional self-concepts from the pre-transition to post-transition, despite a small decrease in self-concepts for students attending Coed classes during the transition year. Sex differences in specific areas of self-concept were unaffected by the transition. Across the 5 years of the study there were no significant differences in either mathematics or English achievement. Results suggest that the transition to Coed benefited both boys and girls in multpile dimensions of self-concept and that these benefits were not gained at the expense of academic achievement. (ABL)