Background: Progress in diagnostic assessment made it possible to diagnose ASD at a young age. Concurrently, intervention research for toddlers with ASD has increased since the past decade. Method: In this study, we report on a five-level meta-analysis of 34 single-subject experimental studies, intended to offer a better insight into what types of interventions are effective for toddlers under the age of three with or at risk for ASD. Results: The analysis revealed a significant positive overall effect size. We found that interventions at home were significantly more effective compared to those in other settings. Other significant moderators regarding intervention characteristics were the agent of intervention and duration in weeks. No moderator effects of study and child characteristics were found. Conclusions: On average, interventions for toddlers with or at risk for ASD are successful. (C) 2017 Elsevier Ltd. All rights reserved.