As the population of emergent bilingual students in the United States continues to grow, it has become increasingly important to ensure that content area instruction is linguistically and culturally inclusive and accessible. Longitudinal survey and state mathematics assessment data were used to examine how the use of students' home language in the English-medium mathematics classroom related to growth in student achievement and attitudes towards mathematics learning. Surveys from 1,274 students attending four middle schools in a Spanish-dominant community reported on teacher and student use of Spanish as well as attitudes towards learning strategies and mathematics. Structural equation modelling revealed that teachers' use of Spanish predicted students' use of Spanish, which was associated with growth in comfort participating in the mathematics classroom. Increased comfort, moreover, was related to growth in mathematics self-efficacy and, in turn, increased interest in mathematics as well as growth on the state mathematics assessment.