Background: This study examined the long-term efficacy of the Incredible Years (IY) BASIC Parenting Programme delivered as a preventive intervention with parents of pre-school children who display signs of attention deficit hyperactivity disorder (ADHD) and conduct problems. Families were followed up after the completion of a controlled trial with 11 Sure Start areas in North and Mid-Wales and North West England. Methods: Participants in the study were 50 pre-school children whose parents had received the intervention. Child ADHD symptoms were assessed at baseline, at follow-up one (6 months after baseline); at follow-up two (12 months after baseline); and at follow-up three (18 months after baseline). Families in the original waiting-list control group were not assessed after follow-up one as they had subsequently received the same intervention. Results: The significant post-intervention improvements in child ADHD symptoms evident at follow-up one were maintained over time, as demonstrated by statistical and clinical stability of measures. No significant differences were found for ADHD symptoms across each follow-up, indicating that the gains made post intervention were maintained for at least 12 months, with 57% of the sample maintaining scores below the clinical cut-off on the Conners. Eighty-six, 58, and 30 per cent respectively had maintained at least a modest, large, or very large improvement in ADHD symptoms at follow-up three. Conclusions: Findings from this study suggest that the IY psychosocial treatment programme is a valuable intervention in the longer term for many pre-school children displaying early signs of ADHD.