Background: Chromosome 18p deletion syndrome (18p-) is associated with a loss of chromosomal material of the short arm (partial monosomy); however, the whole short arm is lost in the majority of cases. The frequency of 18p- syndrome is 1:60000. The syndrome is cytogenetically and clinically heterogeneous. The clinical manifestations vary extremely from mild forms with congenital anomalies and developmental delays to severe brain malformations. Rare cases demonstrate epilepsy and autism spectrum disorders. The deletion breakpoints are also variable. Accordingly, the syndrome needs the analysis of large groups of diseased children by current genomic technologies. Aim of the study: The evaluation of cytogenetic and molecular-cytogenetic technologies for defining critical breakpoints and possible phenotype-genotype correlations. Results: Here, we describe our observations of 15 patients (9 boys and 6 girls) with 18p deletion syndrome, revealed in a large cohort of patients (n=8536). The mean age was 5.1 years; the sex ratio was in favor of boys (1.5:1) in contrast to the literature data. Critical breakpoints associated with this syndrome within the short arm of chromosome 18 were not revealed. It is possible that the clinical features of the syndrome are associated with many breakpoints in chromosome 18 short arm (p11.1->pter). The frequency of 18p- syndrome in children with intellectual disability, developmental delays, and congenital anomalies was 0.2%. The diagnostic aspects of this pathology and the value of molecular cytogenetic methods in studying the syndrome are discussed. Conclusion: We highlight personalized approach to diagnosis of the syndrome for correct genetic counseling for the improvement the life quality and establishing phenotype-karyotype correlations.