Abstract: This study investigated the influence of school-sex on the quality of output from secondary schools in Ekiti State, Nigeria. As a descriptive research, the study population comprised all the 141 secondary schools that presented students for the 2003 senior secondary certificate examinations in the State. Out of the population, a sample of 113 schools (80% of the population) was drawn through the process of stratified random sampling technique. The instrument used to collect data for the study was an inventory. Data were also collected on the performance of students in 5 core subjects in schools’ curriculum namely English Language, Mathematics, Physics, Chemistry and Biology. The data collected were analysed through the use of percentages and the t-test. The findings show that quality of output from single-sex schools was better than the quality of output from mixed sex schools. Single-sex schools outperformed mixed sex chools in the examinations. The mean scores for students in single-sex schools were greater than those of mixed schools. Considering these findings, it was recommended that the State Government should establish more single-sex schools in the State when funds are available. Government should also create single-sex classes in mixed schools or convert some of the mixed schools especially in urban areas into single-sex schools.