RESUMEN
A cross sectional study was conducted on 420 mentally retarded children at school age [6-18 years], chosen from seven institutes for mental retardation in Cairo. The results indicated that consanguinity played an important role as a cause of mental retardation as the figure of consanguineous marriage in this study was [44.5%] and the first cousin marriage comprised [42%]. In addition there were statistical significant differences between: sex, origin, social class, birth order, family history of the same condition, and mental retardation. The relationships between parental age during conception, environmental factors [teratogenic agents, chronic diseases of the mothers prenatal natal, and post natal complications] and mental retardation were found to be statistically significant among non consanguineous group. Finally, some recommendations were suggested to reduce consanguineous marriage and its sequalae of handicaping