This study examined a large, nationally-representative sample of eighth-grade students in the US to assess how the frequency of group learning activity in mathematics classrooms was associated with mathematics achievement and three measures of attitudes toward mathematics. Results showed that moderate amounts of group work in class had positive effects on mathematics achievement scores. Moreover, a significant interactive effect was evident, with girls showing greater achievement benefits from group work than boys. Increased levels of group work were also associated with more positive attitudes toward mathematics.