We report the results of a pilot trial of an evidence-based treatment--Parent-Child Interaction Therapy (PCIT; Eyberg et al. "Psychopharmacology Bulletin", 31(1), 83-91, 1995) for boys aged 5-12 with high functioning autism spectrum disorders and clinically significant behavioral problems. The study also included an investigation of the role of shared positive affect during the course of therapy on child and parent outcomes. The intervention group showed reductions in parent perceptions of child problem behaviors and child atypicality, as well as an increase in child adaptability. Shared positive affect in parent child dyads and parent positive affect increased between the initial and final phases of the therapy. Parent positive affect after the first phase was related to perceptions of improvement in problem behaviors and adaptive functioning.