Inclusion Criteria:  Male or female child with a genetically confirmed diagnosis of PWS (patients can be enrolled if the genetic subtype is not available at inclusion, but the genetic subtype needs to be confirmed during the study); The parents (or legal representative) must have signed the consent form; Treated cohort: the child participated in the OTBB3 study and is aged 16±4 months at inclusion, Untreated cohort: the child has never received OT, is aged 30±6 months at inclusion (in order to maximise the number of children in the untreated cohort) and is followed in France.  Exclusion Criteria:  Administrative problems:  Inability for the parents (or legal representative) to understand/fulfil study requirements; No coverage by a social security regime; Refusal of parents (or legal representative) to sign the consent form;