The aim of this study was to observe whether the independent presence of Sluggish Cognitive Tempo (SCT) directly impacts on the Executive Function (EF), and to determine whether there are deficits in EF that are unique to ADHD predominantly inattentive (ADHD-I) or SCT. Seventy-six participants aged 6 to 17 years and their parents were assessed using a diagnostic interview, an instrument that assesses the EF, and another instrument that assesses the SCT. Two hierarchical linear regression models were performed. The first one analyzed the independent relationship between SCT and EF, and the second model added the symptomatology of ADHD-I. The SCT has a statistically significant direct relation on the EF deficits and remains in the second model even with the inclusion of the ADHD-I. The SCT and ADHD-I symptoms independently promote executive deficits. Children with ADHD-I symptoms showed deficits in most areas of the EF. Deficits in inhibition and initiative are unique to inattention.