This study examined the effects of instructional intensity on the acquisition of storybook vocabulary in first graders who were at risk of early reading failure. It also measured whether the intervention was effective for closing the vocabulary knowledge gap between students who were at risk and their average-achieving peers. A total of 66 students participated in the study, 44 identified as at risk and 22 as average-achieving peers. Students identified as at risk were randomly assigned to either a booster group, where they received explicit instruction of selected storybook vocabulary, or a no booster group, where they received vocabulary instruction in the context of read alouds through an experimental Read Aloud curriculum. A repeated measures design was employed to compare the effects of the intervention. Results indicate that students in the booster group significantly outperformed the students in the no booster and average-achieving peers groups on storybook vocabulary measures. (Contains 3 tables and 2 figures.)