The paper presents the analysis of clinical and laboratory research of one of the rarely chromosomal pathology, which occur in children - Phelan-MacDermid syndrome. The microdeletion of the long arm of chromosome 22 (del(22)(q13)) and the ring chromosome 22 is the main reason of this syndrome. Genetic counselling was conducted in The Center of medical genetic children hospital “OKHMATDYT”. Three children were examined - 1.5, 7.5, and 18 months, with a diagnosis on admission - mental retardation. Genetic (clinical and genealogical, cytogenetic, including molecular cytogenetic (FISH), biochemical and instrumental examination methods were used. The article presents the sample of teamwork, which consists of a clinical and laboratory part of a syndromological diagnosis. Clinical manifestations of the syndrome are not specific. The most frequent of them is muscular hypotonia, (starting from the neonatal period), stato-kinetic and psycho-speech developmental delay and facial dysmorphism. These signs are needs for medical genetic counselling holding and cytogenetic laboratory diagnostic using the FISH-method (determination of the microdeletion of the long arm of chromosome 22, the ARSA locus). In the absence of the syndrome, it is necessary to differential diagnostics with another unbalanced chromosomal pathology (Prader-Willi syndrome, Angelman, Smith-Magenis and velocardiofascial syndrome) and another genetic syndrome (FraX, Smith Lemli Opitz etc.).