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Mental retardation (MR) is a frequent manifestation in patients referred to departments of medical genetics (OLG). At the OLG in Martin their number in the years 1981-1985 was 324, i.e. 21.22% of the total number of examined subjects. MR was found as one of the pathological symptoms (symptomatic MR) in 86.73% and as the only pathological manifestation (isolated MR) in 13.27%. Genetic factors were revealed in 59%, exogenous ones in 19%, and in 22% the aetiology was not unequivocally resolved. As to genetic factors, the most frequent cause were chromosomal aberrations (in 104 patients-53%), a monogenic character was found in 68 subjects (35%) and a multifactorial one in 24 (12%). As to chromosomal aberrations, in 102 cases autosomes were affected (91 times numerical and 11 times structural affection), in four subjects a numerical anomaly of genosomes was involved and once a combined aberration of an autosomal and gonosomal character. The authors give the character and number of different types of aberrations and the incidence of so-called chromosomal markers (12 cases) and they evaluate their causal relationship with MR. Further advances in the aetiological evaluation of genetic factors will be made possible by the introduction of strip methods with a high resolution technique (use of prophasic chromosomes), combined with hybridization in situ, cytogenetic methods for the detection of individuals with the fragile X-chromosome syndrome, and in particular the application of the technology of recombinant DNA for the diagnosis of clinical and genetic units at the gene level.