A movement away from single-sex education occurred in the 1960s and 1970s, just as research was beginning to document positive effects of women's colleges. There has been very little investigation of single-sex education at the secondary level, however. In this study, we compared the effects of single-sex and coeducational secondary schooling, using a random sample of 1,807 students in 75 Catholic high schools, 45 of which were singlesex institutions, drawn from High School and Beyond. Whether concerning academic achievement, achievement gains, educational aspirations, locus of control, sex role stereotyping, or attitudes and behaviors related to academics, results indicate that single-sex schools deliver specific advantages to their students, especially female students. In the recent focus on American secondary education, the relation between school organization and students' academic performance has been looked at critically. What has been considered by some to be an anachronistic organizational feature of schools may actually facilitate adolescent academic development by providing an environment where social and academic concerns are separated. Perhaps a second look at this disappearing school type is warranted.