Parent-child interaction therapy (PCIT) is a form of behavioral therapy to deal with various behavioral problems in children. It works directly with parents and their home interactions with their children. In this paper the application of PCIT to a seven-year-old girl with oppositional defiant disorder (ODD) and her three-year-old sister with behavioral problems is described. A single-case A-B design with concurrent baseline was used. The positive behaviors and problems were assessed for both young girls using self-monitoring and parent video recordings during playtime and home interactions. The treatment was developed in 12 sessions for 3 months. A progressive evolution was observed in both girls’ baseline and in pre- and post-treatment measures of the problem behaviors. Finally, there was a reduction of stress and an increase in the self-efficacy of the parents’ abilities.