"Emotion Regulation" is a key factor in the psychological well-being of children on the autism spectrum. Therapeutic "co-regulation" is posited here as fundamental to addressing Emotion Regulation needs. A therapeutic combination of "Child-Centred Play Therapy" and "Rhythmic Relating" is assessed in its potential to improve "Emotion Regulation" outcomes (over 10 months; 25 weekly sessions), via co-regulatory experience, in a case study of a 5-year-old autistic boy. The parent-rated "Emotion Regulation Checklist" (Shields & Cicchetti, "Developmental Psychology," 33, 1997, 906) was completed every week. Mean "Start" and "Finish" scores were taken for checklist total and all subscales. "Emotion Regulation" percentage-change-over-time values are calculated for the participant and, using percentiles, are compared and statistically ranked in relation to our comparison group: "Autistic boys without concurrent cognitive impairment," "not receiving psychological or behavioural therapies" (over 10 months; N= 66). A graphical representation of Emotion Regulation demonstrates weekly change. As compared to our "comparison group," our participant's overall "Emotion Regulation" improvements (over 10 months) are statistically "likely" to be due to his participation in therapy (p< 0.04). Graphical representation of "Emotion Regulation" over time demonstrates a compelling blind correlation between the period of significant positive change in our participant's Emotion Regulation "and" the therapist's clinical notes on positive change observed in the clinic. [This is the online version of an article published "Counselling and Psychotherapy Research."]