We examined the relations between teachers' use of comments during book reading sessions in preschool classrooms and the vocabulary growth of children with low and moderately low language ability. Using data from a larger randomized controlled trial, we analyzed comments defined as utterances that give, explain, expand, or define. Comments were coded for strategies, which were distinguished by the amount of cognitive distancing required for understanding. Strategies were divided into three levels: low, medium, and high. Videos of whole-class book reading sessions conducted in the fall were transcribed and analyzed for 489 children attending the classrooms of 52 Head Start preschool teachers. Descriptive analyses revealed that teachers largely used medium-level strategies, but relatively small amounts of low- and high-level comments. Logistic regressions revealed relationships between curriculum condition and teachers' use of instructional strategies, such that those assigned to the intervention curriculum used more high-level strategies. Multilevel models found significant relationships between medium-level comments and children's receptive vocabulary growth across one year of Head Start preschool instruction, such that children in classrooms where teachers used more medium-level comments experienced greater growth than those hearing fewer. No moderating effects were found based on children's initial language abilities. Implications for practice and future research are discussed. (author abstract)