Background: Evidence-based psychosocial interventions such as parent training programs are strongly recommended as first-line treatment for preschool-age children with or at-risk of attention deficit/hyperactivity disorder (AD/HD). Objective: Evaluate the effectiveness of the Incredible Years Basic Parent Training (IY) in hyperactive and inattentive behaviors of Portuguese preschoolers. Methods: One hundred children, between three and six years-old, with AD/HD behaviors, who were part of a larger randomized controlled trial in which participants were allocated to either an intervention or control group. In this subsample analysis, there were 52 participants in the intervention condition (IYC) and 48 in the waiting-list control condition (WLC). Multi-informants and multi-measures of child and parenting behaviors were taken before and after the 14-week intervention. Results: Medium-to-large intervention effects were found in primary caregivers' reported measures of children's AD/HD behaviors and on self-reported parenting practices. Independent observations indicated significant short-term effects on positive parenting and coaching. Primary caregivers had a high attendance rate and reported high satisfaction with the program. Additionally, 43% of children in the IYC clinically improved in the primary AD/HD outcome measure, compared with 11% in the WLC. Conclusions: Preliminary results suggest that IY parent training seems to be an effective tool, making the difference in the behavior of Portuguese preschoolers with early signs of AD/HD and their mothers.