Background: Impairments in executive functions (EF) are consistently associated with attention deficit hyperactivity disorder (ADHD) and to a lesser extent, with disruptive behavior disorder (DBD), that is, oppositional defiant disorder or conduct disorder, in school-aged children. Recently, larger numbers of children with these disorders are diagnosed earlier in development, yet knowledge about impairments in clinically diagnosed preschool children and the role of comorbidity is limited. Therefore, the aim of the current study was to examine EF in clinically referred preschool children with a clinical diagnosis of ADHD, DBD and ADHD + DBD. Method: Participants were 202 children aged 3.5-5.5 years, 61 with ADHD only, 33 with DBD only, 52 with comorbid ADHD + DBD and 56 typically developing children. Five EF tasks were administered. Results: Confirmatory factor analysis showed that the two-factor model (inhibition and working memory) fit the data better than a one-factor model in this clinical sample. Preschoolers with ADHD displayed inhibition deficits, also after controlling for IQ. Likewise, preschoolers with DBD displayed impaired inhibition, but when IQ was controlled differences were carried mostly by the effect on the task where motivational demands were high (i.e. when tangible rewards were used). This pattern was also found in the interaction between ADHD and DBD; impaired inhibition in the comorbid group, however, was more severe than in the DBD group. Regarding working memory, few group differences were found. Conclusions: Clinically diagnosed preschool children with ADHD showed robust inhibition deficits, whereas preschool children with DBD showed impaired inhibition especially where motivational incentives were prominent. Severity of inhibition impairment in the comorbid group was similar to the ADHD group.