Deficits in social skills are common in children with Autism Spectrum Disorder (ASD), and there is an urgent need for effective social skills interventions, especially for improving interactions with typically developing peers. This study examined the effects of a naturalistic behavioral social skills intervention in improving social initiations to peers through a randomized controlled trial. Analyses of multimethod, multi-informant measures indicated that children in the active group (SIMI) demonstrated greater improvement in the types of initiations which were systematically prompted and reinforced during treatment (i.e., behavior regulation). Generalization to joint attention and social interaction initiation types, as well as collateral gains in broader social functioning on clinician- and parent-rated standardized measures were also observed.