Researchers and clinicians are recognizing that a subgroup of children with Down syndrome (DS) also present with clinically significant impairments in social-communication and restricted patterns of behavior, such as those that characterize autism spectrum disorders (ASD). Efforts to better describe the behavioral presentation of children with co-occurring DS and ASD could be useful in developing screening and assessment procedures, which could, in turn, facilitate prompt access to appropriate intervention and supports. This study examines the specificity of various forms of maladaptive behavior (e.g., disruptive, repetitive, etc.) in children with co-occurring DS and ASD (n = 22), relative to children with DS (without ASD; n = 54) and those with a nonspecific developmental disorder (DD; n = 34). Parent report of child behavior was obtained on 110 children, ages 3-11 years (mean age = 73 months), who had completed a comprehensive diagnostic evaluation for autism. Two experienced clinical psychologists reviewed more than 50% of cases and independently determined ASD status. One-way analyses of variance were conducted to examine behavioral variables across diagnostic groups. Children with DS + ASD were reported to exhibit more problem behaviors than children in the DS group (without ASD) as well as more self-absorbed behaviors and poorer social relating. The DS + ASD group and DD group had significantly higher scores than the DS only group on overall repetitive behaviors and on "low forms" of repetitive behaviors. Children with DS + ASD were not significantly different from children in the DD group on any of the behavioral variables. Implications for screening and assessment of autism symptoms in children with DS are discussed. (Contains 1 figure, 3 tables and 1 note.)