Evidence shows that the Matthew effect is a persistent problem among early education interventions. The current study examined the degree to which the "ExCELL" ("Ex"ceptional "C"oaching for "E"arly "L"anguage and "L"iteracy) language and literacy professional development intervention for Head Start preschool teachers, shown in prior research to improve teacher quality and increase preschoolers' vocabulary skills, narrowed the vocabulary disparities between children with higher and lower knowledge at preschool entry. Research Findings: Results of a randomized control trial showed that the "ExCELL" program was more effective than a business-as-usual Head Start model at increasing the skills of children with the most limited vocabulary and reducing the Matthew effect, closing the gap between children with the lowest and highest initial vocabulary skills over the preschool year. Moreover, classroom instructional process quality in "ExCELL" settings was a key factor in this achievement. Practice or Policy: When teachers are trained to expose young children in poverty to high-quality classroom instruction (especially regarding process quality), preschoolers can learn substantial amounts of new vocabulary, and those with the lowest initial skills can begin to catch up to their more knowledgeable peers. (Contains 3 tables and 2 figures.)