This study is a prospective, multicentre, interventional cohort study in children with Prader-Willi Syndrome (PWS) over 4 years (no treatment administered). The duration of the preceding OTTB3 study is 26 weeks. An untreated cohort of children with PWS will be included at an age of 2 years and followed up until an age of 4 years.

Regarding the untreated cohort, children with PWS born in France and too old to be recruited in OTBB3 trial, principally those who were born within one year before the start of OTBB3 trial, will be offered to participate in this study. Infants born later who couldn't be included in OTBB3 study will be also offered to participate.

Inclusion Criteria:

1. 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);
2. The parents (or legal representative) must have signed the consent form;
3. Treated cohort: the child participated in the OTBB3 study and is aged 16±4 months at inclusion,
4. 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:

1. Administrative problems:

   1. Inability for the parents (or legal representative) to understand/fulfil study requirements;
   2. No coverage by a social security regime;

2. Refusal of parents (or legal representative) to sign the consent form;