Purpose: Cross-informant ratings are considered best practice for assessing children with autism spectrum disorder (ASD). However, informant disagreement often occurs, which can pose significant challenges to various aspects of clinical services. This study explored the degree of parent and speech-language pathologist (SLP) agreement on ratings of challenging behaviors and social communication skills in preschool children with ASD. Method: Fifty-eight informant ratings of challenging behaviors and social communication skills were collected from parents and SLPs on the same 29 preschool children with ASD (M = 49.93 months, SD = 11.67 months) using the Pervasive Developmental Disorder Behavior Inventory. Parent versus SLP group rating comparisons were assessed with paired t tests and Cohen's d effect sizes. Intraclass correlation coefficients were calculated to examine interrater reliability between individual parent and SLP ratings. Bland-Altman plots were generated to evaluate informant agreement across the entire range of Pervasive Developmental Disorder Behavior Inventory composite scores. Results: Group comparisons indicated that parents rated arousal regulation problems as more severe than SLPs, with no other group differences observed. Parents and SLPs exhibited poor agreement on ratings of challenging behaviors; however, moderate to good agreement was observed for social communication ratings. Conclusions: These results highlight the importance of including parents in the assessment and treatment planning process for preschool children with ASD, as parents may report key behavioral concerns that clinicians may not otherwise observe. Understanding behaviors that may be more prone to informant disagreement has implications for promoting a shared understanding of behavioral concerns and treatment targets between parents and clinicians.