Purpose: The purpose of this study was to determine the effectiveness of a conversation intervention including 500 min of linguistically and cognitively complex talk on the expressive vocabulary growth of prekindergarten children. Method: Children (N = 73) were randomly assigned to control or a 10-week experimental intervention condition. Twice weekly, children in the intervention condition received 25 min of intensive conversation with an adult emphasizing use of rare words, linguistic recasts, and open-ended questions. Expressive vocabulary was measured using the Expressive Vocabulary Test (EVT; Williams, 1997) and lexical diversity obtained through a language sample. Results: Children in the intervention group showed greater growth on the EVT than controls. Children in the intervention group with low vocabulary at pretest also showed greater growth in lexical diversity than controls. Clinical Implications: Findings suggest that relatively small amounts of linguistically and cognitively complex conversation with a trained adult can be a useful strategy for improving the expressive vocabulary skills of children with low vocabularies.