Parent and clinician agreement regarding early behavioral signs of Autism Spectrum Disorder (ASD) in children from a high-risk cohort (siblings of children diagnosed with ASD, n = 188) was examined. Infants were assessed prospectively at 12 and 18 months of age using the clinician administered Autism Observational Scale for Infants (AOSI) and the Autism Parent Screen for Infants (APSI) and underwent a blind independent diagnostic assessment for ASD at 36 months of age. Direct comparison of parent and clinician ratings showed poor agreement on all early behavioral signs, with parent-reported symptoms being better able to differentiate between children with and without ASD at both 12 and 18 months of age compared to clinician observations during a brief office visit. The results suggest that parents may detect some clinically informative behaviors based on their day-to-day observations more readily than do clinicians during brief clinical assessments, a result that needs to be replicated in a non-sibling cohort. Autism Res 2018, 11: 539-547. © 2018 International Society for Autism Research, Wiley Periodicals, Inc.