OBJECTIVES: Many studies have demonstrated comorbidities and overlapping symptoms in attention-deficit hyperactivity disorder (ADHD) and oppositional defiant disorder (ODD). The purpose of this study was to examine the functional impairment in subjects with ADHD, ODD, and in those with both ADHD and ODD. METHODS: 172 male subjects, aged 6 to 15 years old, were enrolled in this study. Based on diagnoses made by applying the Kiddie-Schedule for Affective Disorders and Schizophrenia-Present and Lifetime (K-SADS-PL), the subjects were categorized into four groups: ADHD group (n=64), ODD group (n=17), ADHD+ODD group (n=28), and control group (n=63). The Child and Adolescent Functioning Impairment Scale (CAFIS) was used to measure the functional impairment of the subjects. CAFIS consists of four subscales : Family relationship, Teacher relationship, Peer relationship, and Academic achievement scales. A high CAFIS score implies high functional impairment. Analysis of covariance was conducted to compare the scores between the four groups. RESULTS: Both the ODD and the ADHD+ODD groups had significantly high scores for Parent relationship compared to that of the ADHD group. Compared to the control group, both the ADHD and the ADHD+ODD group had significantly higher scores for Peer relationship and Academic achievement, whereas, the ODD group showed no significant difference from the control group on those two subscales. CONCLUSION The present study showed that subjects with ADHD and ODD have different functional impairment characteristics. The subjects' relationships with their parents were worsened by the presence of ODD. Peer relationships and Academic achievements were significantly affected by the presence of ADHD.