Vocabulary is a prominent contributor to reading comprehension. However, little is known about how students with disabilities with lower vocabulary performance respond to Tier 1 instruction, or how their vocabulary knowledge relates to their content acquisition. To examine the differential impact of Tier 1 instruction for U.S. middle school students with disabilities and to investigate the relations among academic vocabulary, content knowledge, and reading comprehension, this study analyzed data collected in previous randomized controlled trials. Findings suggest that evidence-based Tier 1 instruction targeting academic vocabulary can help eighth graders with disabilities grow as much as students without disabilities on academic vocabulary and content reading comprehension. In addition to students' baseline performance, learning gains in academic vocabulary significantly predicted the growth of content acquisition among students with disabilities.