A cluster randomized trial estimated the effects of a supplemental vocabulary program, Elements of Reading[R]: vocabulary on student vocabulary and passage comprehension in moderate- to high-poverty elementary schools. Forty-four schools participated over a period spanning 2 consecutive school years. At baseline, 1,057 teachers and 16,471 students from kindergarten, first, third, and fourth grade participated. The schools were randomly assigned to either the primary or intermediate grade treatment group. In each group, the nontreatment classrooms provided the control condition. Treatment classrooms used the intervention to supplement their core reading program, whereas control classrooms taught vocabulary business-as-usual. The intervention includes structured, weekly lesson plans for 6 to 8 literary words and aural/oral and written language activities providing multiple exposures and opportunity for use. Hierarchical linear modeling was used to estimate both proximal (Year 1) and distal (Year 2) effects on vocabulary and passage comprehension. The intervention had positive and statistically significant proximal effects but no statistically significant distal effects. The results indicate that the intervention can improve targeted vocabulary and local passage comprehension, but expecting global effects may be overly optimistic. (Contains 13 tables and 2 footnotes.)