The results of a cytogenetic study performed in 523 patients, who were under 18 years of age, were analyzed in order establish the possible causative role of chromosomal anomalies in the occurrence of congenital malfornations with or withuot mental retardation. Conventional staining techniques and chromosomal identification techniques (G and C banding) were used to study the chromosomes. Abuot 48 percent (252)of the total number of patients showed chromosomal aberrations. From these patients, 84.1 percent (212) had complete aneuploidy, homogeneuos or in mosaic (164 corresponded to autosomes and 18 to sexual chromosomes). The remaining 15.9 perent preseted structural aberrations. Our results showed a significant association between abnormal karyotype an malformations with mental retardation