OBJECTIVES: This study was performed to verify and compare deterioration of executive functions in the children with Tourette's disorder (TS) with those with Attention Deficit Hyperactivity Disorder (ADHD) and to understand the differentes. METHOD: We administered Kaufman Assessment Battery for Children (K-ABC), Continuous Performance Test (CPT), Wisconsin Card Sorting Test (WCST) on three groups of 18 children each: tow groups of outpatients, one pure TS and one pure ADHD, diagnosed using DSM-IV criteria, and the third, a control group of normal, healthy children who were selected considering age, sex and academic achievement. The children performed all testings without the use of medications RESULTS: In comparisons of CPT perfotmance, the ADHD group showed poor performance with significant difference in commission error, attentiveness and risk taking (p< .05), and the TS group showed significant difference in attentiveness (p< .05) when compared with the control group. There was significant difference between the ADHD group and the TS group in commission error (p< .05). In comparisons of WCST, the ADHD group and the TS group showed significantly poorer performance than the controls in number of trials administered, total number of errors, conceptual level response, number of categories complete, perseverative response and perseverative error (p< .05), but only the ADHD group showed poor performance in perseverative response and perseverative erro r (p< .05). When the results of CPT were analyzed after adjustment for IQ, the ADHD group showed significantly more errors than the TS disorder group in commission error, and only the ADHD group showed significant difference in attentiveness compared with the normal controls (p< .05). In WCST, while these two groups showed significant difference in number of trials administrated, only the ADHD group showed significantly poor performance compared with the normal children group in the 5 sub-categories such as total number of errors, perseverative response, perseverative error, conceptual level response and number of categories complete (p< .05). CONCLUSION: The abnormalities of executive functions appear in both ADHD and Tourette's disorder, but more apparent abnormalities in executive functions were shown in ADHD than in Tourette's disorder. Also the differences of executive functions suggest that these two diseases may have different psychopathophysiology in these dimensions.