This is a descriptive case study of a 6-years old girl diagnosed with ADHD, whose family received the playful attachment-based intervention – Theraplay. Before therapy the parents felt exhausted, they could not regulate the girl, who developed symptoms of relational trauma. The article includes a theoretical background based on attachment- and polyvagal theory. Assessment before and after treatment are included and there is a description of the parents and the psychotherapists’ experiences during treatment based on interviews in the final phase of the therapy. The results were analysed theoretically and showed how the child’s relational trauma behaviour decreased and how the attachment behaviour improved. Through feelings of safety, physio-emotional meetings, social engagement, guidance and playfulness the stress levels of the mother were reduced and the parents mentalizing ability became stronger. The study shows the importance of working therapeutically with parents, to give parents and child new emotional experiences of being together and to help parents to develop a deeper understanding of their child. Theraplay has previously been successful in treating internalized problems but this case illustrates that attachment-based therapeutic work can be successful in treating children with ADHD.