Recently published social science research suggests that students attending schools with concentrations of disadvantaged racial minority populations achieve less academic progress than their otherwise comparable counterparts in more racially balanced or integrated schools, but to date no meta-analysis has estimated the effect size of school racial composition on mathematics outcomes. This metaregression analysis reviewed the social science literature published in the past 20 years on the relationship between mathematics outcomes and the racial composition of the K-12 schools students attend. The authors employed a two-level hierarchical linear model to analyze the 25 primary studies with 98 regression effects. Results indicate that school racial isolation has a small statistically significant negative effect on overall building-level mathematics outcomes. This relationship is moderated by the size of the sample in the study and by the way the independent variable was operationalized. Although it is small, the effect size is substantively meaningful. The effects are stronger in secondary compared to elementary grades, and racial gaps widen as students age. The emergence and widening of the race gaps as students move through the grades suggest that the association of racial segregation with mathematics performance compounds over time. Implications for educational policy and future research are discussed. (Contains 6 tables and 1 note.)