The effectiveness of the Incredible Years Basic parent programme (IYBP) in reducing child conduct problems and improving parent competencies and mental health was examined in a 12-month follow-up. Pre- to postintervention service use and related costs were also analysed. A total of 103 families and their children (aged 32–88 months), who previously participated in a randomised controlled trial of the IYBP, took part in a 12-month follow-up assessment. Child and parent behaviour and well-being were measured using psychometric and observational measures. An intention-to-treat analysis was carried out using a one-way repeated measures ANOVA. Pairwise comparisons were subsequently conducted to determine whether treatment outcomes were sustained 1 year post-baseline assessment. Results indicate that postintervention improvements in child conduct problems, parenting behaviour and parental mental health were maintained. Service use and associated costs continued to decline. The results indicate that parent-focused interventions, implemented in the early years, can result in improvements in child and parent behaviour and wellbeing 12 months later. A reduced reliance on formal services is also indicated.