This study examined the range, distinctiveness, and correlates of maladaptive behavior in 146 subjects with 5p- (cri du chat) syndrome using the Aberrant Behavior Checklist as a standardized measure. Hyperactivity was the most significant and frequent problem in the sample. Subjects with 5p- syndrome also showed aggression, tantrums, self-injurious behavior, and stereotypies; some of these problems were more pronounced in individuals with lower cognitive-adaptive levels, as well as in those with histories of previous medication trials. Autistic-like features and social withdrawal were more characteristic of individuals with translocations as opposed to deletions, even when controlling for the lower adaptive level of the translocation group. These findings encourage further research on the behavior of individuals with 5p- syndrome.