Comprehensive evaluations include data from multiple informants, but discrepancies occur on adaptive skill measures of youth with autism spectrum disorder (ASD) and intellectual disabilities (ID). This study investigated discrepancies between parent-teacher ratings on a measure of adaptive behavior ["Adaptive Behavior Assessment System, Third Edition" (ABAS-3)] in a clinical sample of 115 youth. Agreement between informants was determined and then discrepancies were identified using paired-sample "t"-tests for the whole sample and subsamples. Factors associated with parent-teacher discrepancies were investigated including age, diagnoses, IQ, autism symptomology, and parent education. Parent-teacher scores were moderately correlated for the ABAS-3 composite and domains. Teachers rated youth with ASD and ID as having higher adaptive skills. Autism symptomology significantly predicted discrepancies between informants on the ABAS-3.