Paediatric Autism Communication Therapy is a parent-mediated, video-aided, communication-focussed intervention for young children with autism spectrum disorder. It has been shown in a UK randomised controlled trial to lead to improvements in parent--child communication and family quality of life, together with a sustained reduction in child autism symptom severity. This qualitative study examined parental perceptions of their participation in Paediatric Autism Communication Therapy within the context of the randomised controlled trial. Semi-structured interviews were carried out with 18 parents within 12 months of completion of the therapy. The thematic analysis provides insights into parents' hopes, expectations, and learning processes when working with Paediatric Autism Communication Therapy therapists and carrying out daily practice in the home. Parents reported positive changes in their interaction and relationship with their child and improvements to their child's communication and interaction. Some also highlighted poignant realisations and emotional challenges associated with taking part in this post-diagnostic therapy. Practical difficulties were also emphasised. Implications for the clinical practice of parent-mediated interventions with young children with autism spectrum disorder are discussed. [This article was written with the PACT Consortium.]