Objective: Parent and teacher ratings of core attention-deficit/hyperactivity disorder (ADHD) symptoms, as well as behavioral and emotional problems commonly comorbid with ADHD, were compared in children with autism spectrum disorders (ASD). Method: Participants were 86 children (66 boys; mean: age = 9.3 years, intelligence quotient [IQ] = 84) who met American Psychiatric Association Diagnostic and Statistical Manual of Mental Disorders, 4th ed. (DSM-IV) criteria for an ASD on the Autism Diagnostic Interview-Revised (ADI-R) and the Autism Diagnostic Observation Schedule (ADOS). Parent and teacher behavioral ratings were compared on the Conners' Parent and Teacher Rating Scales (CPRS-R; CTRS-R). The degree to which age, ASD subtype, severity of autistic symptomatology, and medication status mediated this relationship was also examined. Results: Significant positive correlations between parent and teacher ratings suggest that a child's core ADHD symptoms-as well as closely related externalizing symptoms-are perceived similarly by parents and teachers. With the exception of oppositional behavior, there was no significant effect of age, gender, ASD subtype, or autism severity on the relationship between parent and teacher ratings. In general, parents rated children as having more severe symptomatology than did teachers. Patterns of parent and teacher ratings were highly correlated, both for children who were receiving medication, and for children who were not. Conclusions: Parents and teachers perceived core symptoms of ADHD and closely-related externalizing problems in a similar manner, but there is less agreement on ratings of internalizing problems (e.g., anxiety). The clinical implication of these findings is that both parents and teachers provide important behavioral information about children with ASD. However, when a clinician is unable to access teacher ratings (e.g., during school vacations), parent ratings can provide a reasonable estimate of the child's functioning in these domains in school. As such, parent ratings can be reliably used to make initial diagnostic and treatment decisions (e.g., medication treatment) regarding ADHD symptoms in children with ASDs.