We investigate the impact of a relatively brief cross-curricular intervention, Word Generation, on middle school students' development of taught academic vocabulary. Students (n = 8382) in forty-four middle schools in three urban districts were randomly assigned to treatment or control conditions. Treatment teachers implemented the program with minimal support and varying levels of commitment. Students in treatment schools scored almost a point higher on the curriculum-based vocabulary posttests than those in control schools (Hedges's g = 0.094, p < 0.05). Though there was no main treatment effect on the standardized measures of students' general vocabulary knowledge or reading comprehension, baseline-by-treatment interactions at the school and student level acted to attenuate the Matthew Effect in reading and vocabulary growth.