When children with autistic spectrum disorder (ASD) are assessed in music therapy, significant strengths, potentials and resources emerge that may remain hidden in other, more formalised assessments. Therefore it is becoming more necessary to develop a systematic method of describing this responsiveness, in order to define the expectations of therapy. Music therapy is a particularly important intervention for children with ASD to engage and nurture their capacity for flexibility, creativity, variability and tolerance of change, in order to balance the more structured and behaviourally driven education that is currently popular. The potential of treatment can be defined within the context of meeting healthcare needs. A case study will illustrate a model for defining ‘expectations of therapy’, by drawing on one child’s awakened capacity for interaction through shared, improvised music-making. This paper includes a long appendix providing an evidence based review and recommendations regarding assessment and referral criteria based on current research and clinical evidence.