Prader-Willi syndrome (PWS) is characterized by hypothalamic dysfunction, hyperphagia and a typical behavioral phenotype, with characteristics of autism spectrum disorder (ASD) like stubbornness, temper tantrums and compulsivity. It has been suggested that the oxytocin system in patients with PWS is dysfunctional. In ASD, intranasal oxytocin treatment has favorable effects on behavior.To evaluate the effects of 3 months of twice daily intranasal oxytocin (dose range 16-40 IU/day), compared to placebo, on behavior and hyperphagia in children with PWS.Randomized, double-blind, placebo-controlled, crossover study in the Dutch PWS Reference Center.26 children with PWS aged 3-11 years MAIN OUTCOME MEASURES: (Change in) behavior and hyperphagia measured by Oxytocin Questionnaire and Dykens hyperphagia questionnaire.In the total group, no significant effects of oxytocin on social behavior or hyperphagia were found. However, in boys, the Oxytocin Questionnaire scores improved significantly during oxytocin treatment, compared to a deterioration during placebo (4.5 (-0.8-15.3) vs. -4.0 (-11.3-0.8)), p=0.025). The Dykens hyperphagia questionnaire scores remained similar during oxytocin treatment, while there was a deterioration during placebo (0.0 (-0.8-4.3) vs. -3.5 (-6.0-0.0), p=0.046). Patients with a deletion had significant improvements in both questionnaire scores during oxytocin treatment, but deteriorations during placebo. Oxytocin treatment was well-tolerated and there were no serious adverse events.Intranasal oxytocin treatment has positive effects on social and eating behaviour in 3-11 years aged boys with PWS and in children with a deletion without safety concerns. Intranasal oxytocin in children with PWS might be considered, but individual effects should be carefully evaluated and treatment discontinued if no effects are found.