The present study investigated whether the relationship between mathematics participation and mathematics achievement is reciprocal for boys and girls. In Years 1, 2, 4 and 6 (US grades 7, 8, 10 and 12), we administered mathematics achievement tests to a cohort of 1,495 Flemish students and collected data on the number of classroom hours allocated to mathematics. A cross-lagged panel design was used to analyze the data. Evidence was found for a reciprocal relationship between mathematics participation and mathematics achievement, particularly in Years 4 and 6 (US grades 10 and 12). The results suggest that boys’ better performance in mathematics is related to their higher participation in math, whereas other factors—in addition to gender differences in math achievement—play a role in explaining why boys participate more in mathe-matics than girls.