Mental Retardation (MR), also referred as ‘Intellectual Disability’, ‘Mental Deficit’, ‘Mental Subnormality’ or ‘Mental Handicap’ means delay in mental development; it means an impairment of the intellectual processes of the mind, making it difficult for the person to cope with environment in which they find themselves. The prevalence rate of mental retardation in the general population is estimated to be approximately 1% to 3%. It has been estimated that environmental factors and genetic factors play equal role. Chromosomal abnormalities (numerical and structural) are responsible for up to 28% in mental retardation with the high prevalence of Down Syndrome (DS). In general, over 95% of Down syndrome individuals possess free trisomy 21. Translocations of chromosome 21 (D or G group) were found in 2-4% while 1-2% are mosaics. The present study was aimed to investigate the chromosomal abnormalities in 100 mentally retarded cases. The frequency of regular trisomy was 18 (18%). The frequency of Robertsonian translocations 46, XY, t (21;21) +21 and 46, XY, t (14;21), +21 were 3 (3%) and 1 (1%) respectively. The frequency of mosaic 46, XY/47, XY, + 21 was 2 (2%). Since the study is already published without detailed case studies, the present paper discusses each case in detail.