Many adults with Prader-Willi syndrome are affected by behaviors such as tantrums, skin-picking, and compulsions. The nature and extent of these problems suggest more attention be directed to their emergence in childhood. Our purpose was to investigate behavior problems in children with this syndrome and identify the age at which these behaviors emerge. Parents of children with Prader-Willi syndrome, Down syndrome, and those developing typically completed questionnaires. Children with Prader-Willi syndrome exhibited more compulsions, skin-picking, and tantrums than did the other groups. A discriminant analysis of behavior variables derived two statistically significant functions that were interpreted as developmental milestones and problematic behavior. These functions correctly predicted membership for 79% of grouped cases.