Restricted and repetitive behaviours vary greatly between autistic people. Some are a source of pleasure or create opportunities for learning; others may be detrimental in day-to-day life or cause harm. We have developed, in close collaboration with parents/carers, the Understanding Repetitive Behaviours programme, designed for families of young autistic children, to help them recognise, understand and respond sensitively to their child's impactful restricted and repetitive behaviours. This study is a clinical and cost-effectiveness, multi-site randomised controlled trial of the Understanding Repetitive Behaviours parent programme versus a psychoeducation programme (equivalent to current best practice), learning about autism. Participants were parents/carers, with an autistic child aged between 3-9 years and 11 months. The study was delivered across three sites in England and Scotland. Analyses were completed using intention-to-treat principles. Two hundred and twenty seven families were randomised (113 in LAA; 114 in Understanding Repetitive Behaviours arm). No differences were found between the arms on the primary outcome measure (The Clinical Global Impression - Improvement scale). Analysis of secondary outcomes indicated that children in the Understanding Repetitive Behaviours arm were more likely to be rated as responders in target impactful restricted and repetitive behaviours at 24 weeks but that this effect was not maintained at 52 weeks. Improvements in parent and family functioning were apparent, with no evidence of differences between the arms. The study reconfirms that it is important that clinicians consider both restricted and repetitive behaviours and social communication needs of autistic children with parents when planning appropriate support.Lay abstractAutistic children, frequently repeat the same behaviours over and over, have specific interests or like things to stay the same. These behaviours and interests are often fun and helpful. However, sometimes they can impact negatively on day-to-day life or put the child at risk of harm. Working closely with parents of autistic children, we developed an 8-week programme (Understanding Repetitive Behaviours) to help them recognise and understand these behaviours. This study aimed to find out whether the understanding repetitive behaviour programme was helpful and good value for money. Two hundred and twenty seven families were allocated by chance to receive either Understanding Repetitive Behaviours or a learning about autism programme. When experts made judgements about whether children showed positive changes across various measures, and these were analysed, there were no differences between the programmes. However, parents who attended the Understanding Repetitive Behaviours programme reported improvement in one of their child's specific repetitive behaviour (selected to be the main focus of the Understanding Repetitive Behaviours programme) at 24 weeks after the end of the programme. Parents who attended either programme reported more confidence, greater wellbeing and less stress up to 1 year after the end of the study.