Some U.S. school districts are experimenting with single-sex schooling, hoping that it will yield better academic outcomes for students. Empirical research on the effects of single-sex schooling, however, has been equivocal, with various studies finding benefits, disadvantages, or no effect. Most of this research is marred because families generally choose whether the child attends a single-sex or coeducational school, so there are selection effects that create pre-existing differences between students in the different types of schools. The research reported here capitalized on the case of Korea, where students are randomly assigned to single-sex or coeducational schools. Using 2007 Trends in International Mathematics and Science Study (TIMSS) data, we applied hierarchical linear modeling to account for the nesting of students within schools. Results for eighth graders indicated no differences between students in single-sex and coeducational schools in mathematics and science achievement. Results from the 2003 TIMSS data replicated the finding: students' mathematics and science achievement was unrelated to the gender composition of their school. These results call into question whether single-sex schooling has the academic advantages claimed by its proponents. (Contains 4 tables.)