To evaluate the 12-month efficacy of a parent-based intervention programme on children’s and mothers’ outcomes in a sample of Portuguese preschoolers displaying early hyperactive and inattentive behaviours (AD/HD behaviours), 52 preschool children whose mothers had received the Incredible Years basic parent training (IY) were followed from baseline to 12 months of follow-up. Reported and observational measures were used. Effects were found in the children’s reported AD/HD behaviours at home and at school after 12 months. Large effect sizes were also found in mothers’ variables: a decrease in self-reported dysfunctional parenting practices and an improved sense of competence and observed positive parenting. However, the improvements in coaching skills that have been observed after 6 months of follow-up decreased over time. No other significant differences were found between 6 and 12 months follow-up, with small effect sizes indicating that the significant post-intervention changes in child and parenting measures were maintained. After 12 months of follow-up, there was a clinically important reduction of over 30 % in reported AD/HD behaviours in 59 % of children. The sustained effects observed both for children and their mothers suggest long-term benefits of IY. Therefore, efforts should be made by Portuguese policy makers and professionals to deliver IY as an early preventive intervention for children displaying early AD/HD behaviours.