AbstractIn 122 high-functioning children with autism spectrum disorder (ASD; 9–13 years; 19 girls), we investigated the effectiveness of a 15-session social skills group training (SST) with and without parent and teacher involvement (PTI) in a randomized controlled trial with three conditions: SST (n = 47), SST–PTI (n = 51), and care-as-usual (CAU, n = 24). Hierarchical linear modeling was used for immediate and 6-month follow-up analyses. Measures were administered before randomization (blind), post-treatment and at follow-up (not blind). Trial registration: Dutch Trial Register; http://www.trialregister.nl; NTR2405. At post-treatment, children in both SSTs had improved significantly more than CAU on the primary outcome, Vineland Socialization (SST: Cohen’s d = 0.39; 95% CI − 2.23 to 3.11 and SST–PTI: d = 0.43; 95% CI − 2.19 to 3.15) and on the secondary outcome parent-SSRS “Cooperation” (SST: d = 0.43; 95% CI − 0.23 to 1.15 and SST–PTI: d = 0.45; 95% CI − 0.21 to 1.17), with no difference between post-treatment and follow-up. Additionally, children in SST–PTI improved significantly more on the teacher-SSRS than in CAU [“Cooperation” d =0.42 (95% CI − 0.33 to 1.13); “Assertion” d =0.34 (95% CI − 0.39 to 1.11); “Self-Control” d =0.61 (95% CI − 0.08 to 1.34)] and in SST [“Cooperation” d =0.34 (95% CI − 0.37 to 1.05); “Self-Control” d =0.59 (95% CI − 0.13 to 1.32)]. The current study corroborates earlier findings in smaller samples and wider age ranges, with small but statistically significant effects of SST for high-functioning pre-adolescent children with ASD. Parental and teacher involvement intensified treatment, yet did not yield an additional effect relative to SST for children only, as reported by parents. 6 months after training, no further improvement or decline was found.