The present study examines co-occurring psychiatric syndromes in a well-characterized sample of youths with autism spectrum disorders (ASD; n = 177) and their siblings (n = 148), reported independently by parents and teachers. In ASD, parents reported substantial comorbidity with affective (26%), anxiety (25%), attentional (25%), conduct (16%), oppositional (15%), and somatic problems (6%). Teachers reported a much lower prevalence. Autistic severity scores for children with ASD exhibited moderate correlations with general psychopathology within- but not across-informants, whereas, sibling correlations were significant both within- and across-informants. Results support the role of environmental context in psychiatric symptom expression in children affected by autism and suggest that informant discrepancies may more provide critical cues for these children via specific environmental modifications.