Adolescents with Down syndrome can demonstrate increased behavior problems as compared with typical peers. Few studies have explored whether behavior impacts adaptive functioning. Caregiver report from the Behavioral Assessment System for Children, 2nd Edition (BASC-2; Reynolds & Kamphaus, 2004) and the Child Behavioral Checklist (CBCL; Achenbach, 2001) was collected for 52 adolescents (24 males). Participants were most frequently rated as having problems with inattention and social withdrawal. Attention problems accounted for a significant proportion of variance in Activities of Daily Living (R[subscript 2Adj] = 0.30), Leadership (R[subscript 2Adj] = 0.30), Functional Communication (R[subscript 2Adj] = 0.28), Adaptability (R[subscript 2Adj] = 0.29), and Social Skills (R[subscript 2Adj] = 0.17) on the BASC-2. Convergent validity between the CBCL and BASC-2 was limited. Findings inform the clinical utility of the BASC-2 and CBCL in this population. Improved specification of behavior will facilitate targeted intervention, thus improving outcomes for individuals with Down syndrome and their families.