OBJECTIVE: To study the frequency of chromosomal abnormalities in severely mentally retarded Iraqi patients. Secondly, to determine the types of chromosomal abnormalities that play a major role in the causation of mental retardation and to compare our results with those reported elsewhere. METHODS: Twenty one patients with severe mental retardation were subject to chromosomal analysis. The lymphocyte culture was carried out according to standard methods. RESULTS: Fourteen of the subjected mentally retarded patients had chromosomal abnormalities, 13 autosomal abnormalities, and only one had sex chromosomal abnormalities. However, structural autosomes were found to be the most prominent abnormalities and only 2 patients were demonstrated to have diagnosable syndrome. CONCLUSION: Chromosomal abnormalities are an important cause of mental retardation and its frequency increased with the severity of mental retardation. We concluded that chromosomal studies in mentally retarded patients help in accurate diagnosis and proper prognosis followed by genetic counseling and management rehabilitation.