Prader-Willi syndrome, induced by a loss of function of paternal genes in the subcentrometric region of the chromosome 15 (q11.2q13), is a complex neurodevelopmental disorder with characteristic obesity resulting from hyperphagia. In addition behavioural disturbancies with obsessive-compulsive features, aggression, temper tantrums included, are relatively frequently seen and they often require psychiatric intervention. In this part of the paper we reviewed the recent data of behavioural phenotype the correlations of phenotype-genotype and possibilities of the multidisciplinary support for the affected persons and theirs families.