After a brief review of internationally recognized empirical studies of the relative impact of single-sex and coeducational secondary schooling on girls' performance, the paper provides impact estimates drawn from two educational surveys carried out in a region which has many state-supported single-sex secondary schools, namely, Northern Ireland. Data relating to girls from two pupil cohorts were re-analysed using multilevel statistical software not available to the original researchers whose main concerns are not considered here. Six outcomes were investigated. They were all related to pupil performance in public examinations taken, typically, after five years of secondary schooling. After adjustment for a range of pupil-intake measures and for the selective status of the schools (grammar/nongrammar), a small achievement advantage favouring single-sex schooling for girls is reported. However, this advantage was not statistically significant as far as any one of the six outcome measures was concerned.