Many researchers have illustrated the multi-faceted nature of classroom mathematics discussions, but few have demonstrated the effect of discussion on students' assessment performance. We developed and employed a discussion observation instrument in 20 third and fourth grade classrooms in an economically disadvantaged, linguistically diverse school district and used hierarchical linear modeling (HLM) to determine whether between-class variation in word-problem-test scores can be explained by levels of class discussion. Results suggest overall class discussion scores, as well as two specific discussion features, variety of approaches and opportunities to speak, are significantly related to test performance. These results suggest classroom instruction including high-level math discussion may improve students' performance on written measures of achievement.