This study described the commenting practices of Head Start teachers, and the relationship of comments to the expressive and receptive vocabulary growth of children with below-the-mean language ability across one year of preschool. Participants included 52 Head Start teachers, and 489 children (247 early intervention candidates and 242 Head Start typical). Descriptive analyses reveal that teachers used informative comments that gave or explained information more frequently than comments that responded to children's utterances, and that these comments contained more conceptually focused content than vocabulary or skills content. Responsive and conceptually focused comments were significantly related to the children's receptive vocabulary growth, and were moderated by children's initial language ability indicating the presence of the Matthew Effect. These findings underscore the importance of integrating instructional comments into book reading sessions, and the need to differentiate instruction based on children's initial vocabulary sizes. Practical implications are discussed.