Three questions were addressed using data from the National Educational Longitudinal Study of 1988. First, are there differences between single-sex and coeducational Catholic secondary school students m academic and social psychological outcomes? Second, do these differences especially favor young women in single-sex schools? Third, can student pre-enrollment differences account for apparent sector effects? The results indicated that single-sex Catholic secondary, schools were not especially favorable academic settings and that any advantages of attending these schools only benefited boys. However, any sector differences in student achievement test scores were explained by pre-enrollment differences in measured background and prior achievement. Recent changes in the demographic make-up of all Catholic high schools may account for the differences between our findings and prior research.