This article examines the 11-year Soviet experiment with boys' schools as a way to cast new light on scholarly research and public debates about single-sex education. Drawing on archival and published materials by educators who described school conditions, identified problems, suggested reforms, and evaluated remedies, the author argues that separating boys from girls exacerbated the problems of coeducational schools while generating additional difficulties and dilemmas specific to sex-segregated schools. With the restoration of coeducation in 1954, single-sex schooling, and boys' schools in particular, had been decisively repudiated. For educators, policymakers, and parents engaged in the current debate on sex-separated schooling, this case study involving more than a million male pupils can serve as a cautionary tale about the possible difficulties and potential consequences of a large-scale gender segregation policy. (Contains 6 notes.)