Consanguinity among parents as a cause of mental retardation in their children is debatable. The present study was conducted to find out the effect of consanguinity on mental retardation where the causative factor is not established. A total of 517 mentally retarded persons and their families were studied out of which 160 were born of consanguineous marriage and 357 were of non-consanguineous marriage. The results indicated that, when there is a history of mental retardation in the family and if the parents are consanguineously married, the risk of mental retardation in the offspring is significantly high (chi 2 = 11.52; P less than 0.001). Among the consanguineously married families, the blood relationship of uncle-niece seems to have the highest risk of affecting the offsprings. The implications are discussed in detail.