Middle school students with learning disabilities often struggle with reading and literacy skills, including vocabulary, and require support in content-area classes such as mathematics where weak vocabulary knowledge can negatively affect learning, achievement, and deep understanding. However, in general, middle school mathematics teachers do not receive training in literacy instruction, much less in explicit vocabulary instruction. Additionally, relatively few studies have examined content-area vocabulary instruction at the middle or secondary school level. This study addressed these gaps in the research by examining the effect of a performance feedback and coaching intervention on middle school mathematics teachers' use of explicit mathematics vocabulary instruction. Results of the study indicate that performance feedback and coaching had moderately positive effects on teachers' use and quality of explicit mathematics vocabulary instruction.