The aim of the study was to see whether there were lasting effects of a behaviourally-based group parenting programme when delivered in a real life, regular clinical practice setting. Follow-up was one year after the end of a controlled trial that involved four local child and adolescent mental health services in London and Southern England. The participants were fifty-nine children aged 3–8 years referred with antisocial behaviour, whose parents received the Webster-Stratton Incredible Years basic videotape group programme. Those in the waiting list control group were not followed up as some went on to receive the same intervention. Measures included the semi-structured parent interview (PACS) and questionnaire (SDQ) about child behaviour. At follow-up, the original improvement in the intervention group was found to have persisted, with no loss of treatment effectiveness (effect size compared to pre-treatment score 0.91 standard deviations, compared to post treatment score −0.06 SD). The proportion of children in the clinical range before treatment was 68%, at follow-up 37%. Children with the most severe initial problems changed the most; risk factors such as low income, being a lone parent, or being in an ethnic minority did not reduce treatment effectiveness. Parenting groups can reduce serious child antisocial behaviour effectively in the longer term. This is an important ongoing benefit for the children and their families. If this trajectory continues to be maintained in the future, then the poor long-term prognosis, which includes criminality and social exclusion, is likely to be improved.