Mental retardation (MR), defined by the World Health Organization(WHO) as an intelligence quotient (IQ) <70, is characterized by significantly limited cognitive functioning, coupled with limitations in adaptive skills. Down syndrome is the most common genetic cause of intellectual disability in the population and is due to a gene dosage effect of the presence of an additional chromosome 21(Vundinti etal., 2011) or a partial trisomy, mainly in the 21q22 region (Frias etal., 2002). The main aim of this study is to evaluate the Cytogenetic causes for the occurrence of the mental retardation. The present study was carried out with 100 MR cases and 100 age and sex matched healthy controls from North Coastal Andhra Pradesh. With prior informed consent, 5 ml of blood was collected into sterile heparinized tubes from 100MR cases and 100 healthy controls for evaluation of the chromosomal abnormalities by using Leukocyte Blood Culture and G-banding techniques. Out of 100 mentally retarded people analyzed, 76(76%) had normal karyotype and remaining 24(24%) were Down syndrome. Among them regular free trisomy constituted 18(18%) cases, Robertsonian translocations in 4(4%) cases and mosaicism was recorded in 2(2%) cases. The study confirmed the findings of earlier studies carried out in India and other countries. It emphasizes free trisomy 21 was found to the most  frequent  autosomal aberration  of  Down syndrome when compared with Robertsonian translocations and mosaics.