Executive functioning in 3-year-old preschoolers with attention-deficit hyperactivity disorder (ADHD), oppositional-defiant disorder (ODD), comorbid ADHD+ODD, and children without any of these conditions (control group) was examined. A community sample including 622 children was diagnosed using a diagnostic interview following DSM-IV criteria, and assessed using the Behavior Rating Inventory of Executive Function Preschool version (BRIEF-P) and the Kiddie-Conners’ Continuous Performance Test. The children diagnosed with ADHD showed the poorest executive function (EF) profile in comparison with controls, and were closely followed up in this respect by the comorbid ADHD+ODD children. The ADHD and comorbid groups presented similar executive difficulties. The ODD group obtained mean scores statistically equal to those of controls in EF. These findings suggest that, in preschoolers, executive functioning deficits assessed with a performance-based measure or with behavioural descriptions are specific to children with ADHD, in comparison with those with ODD. This study contributes knowledge about EFs in two prevalent and comorbid disorders in preschool children, ADHD and ODD, knowledge that can help our understanding of specific deficits and the design of specific early intervention initiatives.