The purpose of this study is to examine the behavioral and developmental characteristics of children with inv(9). This case control study included 12 inv(9) subjects and 45 normal students. All of the subjects, together with their parents, underwent a psychiatric interview and parent questionnaire consisting of a Child Behavior Checklist (CBCL). The mean scores with regard to the social problems and total problems profiles of the CBCL were significantly higher in the inv(9) group than in the normal control group. The inv(9) group exhibited language and motor developmental delay. These findings suggest that there is a possibility of inv(9) being associated with child developmental or behavioral problems.