The behavioral phenotype of 47,XXY (Klinefelter syndrome) includes increased risks for developmental delays, language-based learning disabilities, executive dysfunction/ADHD, and socialemotional difficulties. However there is significant variability between individuals with 47,XXY, and many children and adolescents have minimal or no behavioral features while others have quite significant involvement. This paper describes behavioral features in a cohort of 57 children and adolescents with 47,XXY, including results on standardized measures of behavior (BASC-2), attention (Conner's Rating Scales), and social skills (Social Responsiveness Scale). A subset was directly assessed for autism spectrum disorders using the ADOS and ADIR. We discuss our results within the context of previous literature, including implications for genetic counseling, recommendations for care, and areas for future research.