The current study examined the effect of Parent-Child Interaction Therapy (PCIT), a parent-training intervention for child behavior problems, on child language production. Participants were 46 children (ages 20-70 months) with externalizing behavior problems and with or at risk for developmental delay. Parent-child dyads were randomly assigned to a waitlist control or immediate treatment group. Parenting skills learned during PCIT (i.e., "do skills") and children's word tokens and word types were measured at baseline and 4 months later. Findings suggest an indirect effect of parent do skills on the relation between group and child word types, such that more parent do skills predicted more child word types for families receiving PCIT. The present study found that mothers' use of child-directed skills played an important role in the growth and improvement of child language. Results suggest that parent-training interventions targeting child behavior problems may also foster child language production.