This study evaluated the efficacy of the Word Learning Strategies (WLS) supplementary program designed to develop upper-elementary students' vocabulary skills in order to improve reading comprehension. The study used a true, group-randomized, experimental design, which randomly assigned 46 4th grade classrooms (n=1324 students) from 12 districts to a treatment or control group. The results from the first cohort indicate that the program was positively associated with gains in students' vocabulary learning as measured by Word Learning Strategies Test and in students' reading comprehension as measured by Gates-MacGinitie Reading Test, after accounting for differences in baseline test scores. The use of the WLS program also led to increases in teachers' awareness of strategies to support their students' vocabulary and reading comprehension. [This paper was published in the Proceedings of the 2016 American Evaluation Association (AEA) Annual Conference. Atlanta, GA.]