Deficits in academic vocabulary knowledge plague students who struggle with reading and impact their reading comprehension and performance in content area courses. Experts have developed interventions aimed at improving vocabulary instruction. However, questions remain concerning the effects of vocabulary gains on reading comprehension and content learning. To address these questions, we explored the impact of an intervention on social studies academic vocabulary knowledge among 8th-grade students with below-average reading comprehension. Results of a randomized controlled trial indicated that treated students had greater mastery of social studies vocabulary. In addition, scores on vocabulary knowledge predicted content acquisition and reading comprehension scores. These results underscore the need for in-depth vocabulary instruction and the importance of assessing gains in word knowledge in terms of both breadth and depth.