Background: Early intervention in autism spectrum disorder (ASD) improves outcomes, but treatment access depends on early identification. Despite reliability of most autism diagnoses by age 24 months, the majority of children with autism do not receive comprehensive evaluations until after age 36 months. Although many possible reasons exist for delayed evaluations, the referral decision process itself is not clearly defined nor understood. In particular, little is known about how perceptions of early childhood symptoms of ASD interact with decision criteria to ultimately refer a child for comprehensive diagnostic assessment. Method: To better understand pre-referral perceptions and decision factors involved, we provided multiple video clips from early comprehensive autism evaluations as prompts, then asked early childhood clinicians and educators (n = 346) to make decisions about autism referral, then to identify factors considered when making decisions. Results: Decision factors cited most often were play, social interactions/engagement, and verbal behaviors, but none were cited by a clear majority. Some important early signs were infrequently mentioned: absence of joint attention, social reciprocity and gestures. Accuracy of referral decisions was similar across disciplines, indicating a broad range of professionals capable of identifying early signs of autism. Conclusions: Autism training across early childhood professional disciplines should emphasize importance of early signs involving significant absences of behaviors, such as low joint attention, gestures, and social reciprocity in addition to odd social, verbal, and play behaviors. Such targeted training may encourage earlier referrals when autism is suspected in young children. © 2018 Elsevier Ltd