In this quasi-experimental study, we assessed the promise of Word Generation, a research-based academic vocabulary program, on improving the reading achievement outcomes of struggling sixth-grade readers in an after-school small group instructional setting. After 34 hours of academic vocabulary instruction, we compared the performance of a school-identified group of students (n = 36) as needing reading assistance who voluntarily participated in the program to that of a school-recruited comparison group of student peers (n = 36) who did not need reading assistance but participated in a different after-school instructional program. Results show that the vocabulary knowledge and reading comprehension performance of intervention students improved significantly after completing 34 hours of academic vocabulary instruction using the Word Generation curriculum. More importantly, intervention students received nearly identical extended scale scores on the Group Reading Assessment & Diagnostic Evaluation test at the end of the intervention when compared to students in the comparison group after adjusting for initial mean differences. These findings indicate that a modest dose of instruction in cross-disciplinary vocabulary instruction can help close the reading achievement gap between skilled and less-skilled sixth-grade readers.