Parent-child interaction therapy aims to help parents develop strategies to facilitate a child’s fluency, including modifying parent interaction variables. The aim of this paper is to explore whether there is evidence that this therapy has an impact on children’s use of language. Data from twelve children who have participated in two single subject replication studies will be presented. During both studies, the children were video recorded playing with their parents at home before, during and after therapy. The videos were transcribed using Codes for the Human Analysis of Transcripts and the data analysed using Child Language Analysis programme (MacWhinney 2000).