Background: Recent research has shown that the range of repetitive behaviour seen in individuals with Prader-Willi syndrome (PWS) extends beyond food-related behaviour. Methods: The presence and intensity of repetitive, rigid and routinized behaviour in children with PWS was compared with that seen in children with another neurodevelopmental condition in which repetitive behaviour is common: children with autism. Parents completed the Childhood Routines Inventory (CRI). Results: Contrary to our predictions, controlling for developmental level, children with PWS and children with autism showed similar levels of repetitive and ritualistic behaviour overall and on the two CRI factors measuring just right and repetitive behaviour. Indeed, the majority of the sample of parents of children with PWS endorsed most items on the CRI. However there was some specificity at the level of individual items with parents of children with PWS more frequently endorsing an item on collecting and storing objects and parents of children with autism more frequently endorsing lining up objects, has a strong preference for certain foods and seems aware of detail at home. Conclusions: These findings confirm the range of repetitive behaviours that form part of the behavioural phenotype of PWS, including insistence on sameness and just right behaviours, and uncover a surprising overlap with those seen in children with autism. Clinical management for children with PWS should include advice and education regarding management of repetitive and rigid behaviour. Future research should investigate whether the repetitive behaviours that form part of the behavioural phenotype of both PWS and autism are associated with a common neuropsychological, neurotransmitter or genetic origin.