OBJECTIVE:In Chronic Tic Disorders (CTD) associated Attention Deficit Hyperactivity Disorder (ADHD) is very common. Hence, it is important to clarify how both conditions are related to cognitive dysfunctions in patients with CTD+ADHD comorbidity. Recent studies on neuropsychology revealed equivocal results, mostly due to methodological shortcomings like problems in sample composition. Thus better and more detailed information on this topic is needed to improve diagnostic and treatment approaches. METHOD:Three tasks related to different domains of executive functions (the Matching Familiar Figures Test, the Stroop color-word interference task, and a computerized version of the Wisconsin Card Sorting Test) have been performed in two independent samples (altogether n = 138 children) both including four groups of children (CTD-only, CTD+ADHD, ADHD-only, healthy controls) matched for age and IQ. To specify the influence of either tics or ADHD-symptoms on executive functions and to answer the question of their interactive or additive relationship two-way analyses of variance (MANOVA) for the factors CTD (yes,-no) x ADHD (yes,no) were conducted. Eta squared was calculated to reveal the effect sizes for each factor. For a deeper understanding of group differences and to better enable the comparison with data in literature, additional analyses of variance (ANOVA) with posthoc testing were applied. RESULTS:In summary, there was a main effect only for the factor ADHD reflected by decreased performance, while no main effect of the factor CTD could be found. Admittedly, the effects were not uniform in both samples. However, in all three tasks and both samples, uniformly no interaction between the main factors has been observed. CONCLUSIONS:In cases of coexisting CTD+ADHD the factor ADHD shows the main negative impact on neuropsychological performance and this impact seems to be independent of any feature of the coexisting tics (additive model). This supports the notion to primarily treat the ADHD-symptoms in order to increase cognitive self regulatory abilities of these children. Contrarily, tics seem to have little impact on cognitive performance in most cases.