This study provides the first large-scale quantitative exploration of mathematical language use in U.S. classrooms. Our approach employs natural language processing techniques to describe variation in the use of mathematical language in 1,657 fourth and fifth grade lessons by teachers and students in 317 classrooms in four districts over three years. Students' exposure to mathematical language varies substantially across lessons and between teachers. Students whose teachers use more mathematical language are more likely to use it themselves, and they perform better on standardized tests. These findings suggest that teachers play a substantial role in students' mathematical language use.