Sex chromosomal aneuploidy is the most common disorder of sex chromosomes in humans, with an incidence of 1 in 400 newborns. The addition of more than one extra X and/or Y chromosome to a normal male karyotype is less frequent and has its own distinctive physical and behavioral profile. This study examines the behavioral similarities and differences in individuals with 48,XXYY compared to 48,XXXY and 49,XXXXY. The participants include 11 males with 48,XXYY and 13 males with 48,XXXY and 49,XXXXY. Using the Vineland Adaptive Behavior, the Achenbach Child Behavior Checklist, and the Reiss Personality Profiles, parents are asked to characterize the behavior and personality of their boys with sex chromosome tetrasomy and pentasomy. Males with 48,XXYY have higher overall adaptive scales in daily living skills, socialization, and communication compared to males with 48,XXXY and 49,XXXXY. Both groups are at risk for maladaptive behavior, although 48,XXYY males are at a higher risk for internalizing and externalizing symptoms. 48,XXXY and 49,XXXXY function at a lower cognitive level and their behavior is often immature for their chronological age. Both groups display interests in helping others, but have a low tolerance for being rejected or teased. Specific recommendations and interventional strategies are provided for individuals with 48,XXYY, 48,XXXY, and 49,XXXXY. © 2007 Wiley-Liss, Inc.