Many children with autism spectrum disorder (ASD) exhibit difficulties with negative affect. Cognitive behavioural therapy (CBT) has been successfully adapted for individuals with ASD to treat these difficulties. In a wait-list control study, for example, group analyses showed promising results for young children with ASD using a developmentally adapted group CBT approach. This report examined response to group CBT in terms of individual-level change in young children with ASD. Eighteen children with ASD, aged 5–7 years, and their respective parents participated in treatment. Parents completed pre- and post-treatment measures of negative affect and related behaviours. Treatment responders and non-responders were grouped based on significant treatment outcomes as assessed by statistically significant change for lability/negativity and 20% decrease in intensity, duration or frequency of emotional outbursts. Results indicated that 67% of children met criteria as a treatment responder, showing meaningful improvement in at least two outcome measures. No significant group differences emerged for initial characteristics before treatment. Wilcoxon signed rank tests determined pre-/post-treatment change in parental confidence for each treatment responder group. Results indicated statistically significant increase for the treatment responder group in parent-reported confidence in their own ability and in their child's ability to manage the child's anger and anxiety, but these results were not significant for the treatment non-responder group. Results provide additional evidence that CBT can significantly decrease expressions of anger/anxiety in children with ASD as young as 5 years, yet also suggest need for further improvement.