This study examined whether musical and emotional attunement predicts changes in improvisational music therapy with children with autism (4–7 years, N = 101, majority: no/limited speech, low IQ), assessed over 12 months. Attunement, as observed from session videos, and changes in generalized social skills, judged by blinded assessors and parents, were evaluated using standardized tools (Assessment of the Quality of Relationship, Improvisational Music Therapy Principles, ADOS, SRS). In contrast to the smaller pilot, we did not find significant effects between attunement and changes in outcomes, only tendencies in the same direction are observed. Findings suggest that symptom severity is associated with the therapist’s ability to attune to the child. They further raise questions concerning outcome selection and user involvement. Electronic supplementary material The online version of this article (10.1007/s10803-020-04448-w) contains supplementary material, which is available to authorized users.