Background: Mental retardation (MR) affects 2 – 3 % of the general population, it is of heterogeneous etiology, and it is considered to be genetic in 50 ‒ 60 % cases, which is a challenge in the diagnosis of mental retardation. Objectives: To define genotype-phenotype correlations in patients with MR the pediatric population using cytogenetic and highly sensitive techniques of molecular (cyto) genetics (FISH, MLPA, aCGH, NGS) which will increase the rate of detected genomic disorders. Subjects and methods: The observational cross-sectional study included 300 children from 0 to 18 years of age with MR with and without dysmorphia examined in the genetics clinic and genetics department, Clinic for Pediatrics, University Hospital Center Osijek in the period from 2008 to 2018. Descriptive data are expressed in absolute and relative frequencies, ie percentages. New phenotypes and genotype-phenotype correlations are described. Results: The highest number of chromosomal aberrations was determined using cytogenetic techniques in 29 % (87/300) of subjects. FISH technique revealed microdeletion syndromes in 58,6 % (17/29) of subjects, while aCGH determined aberrations in 18,5 % (5/27) of subjects. Molecular genetic analysis of the number of CGG triplets in 8,7 % (11/126) subjects detected fragile X syndrome as a cause of MR. Conclusion: The combination of cytogenetic and molecular techniques revealed a larger number of chromosomal and submicroscopic aberrations in the MR background, which facilitates the definition of new genetic syndromes and correlation of genotypes and phenotypes.