In many regions of Asia and Africa consanguineous marriages--conventionally referring to unions between persons related as second cousins or closer--account for approximately 20 to 50 percent of all unions and, contrary to received opinion, there is no evidence of a significant decline in their popularity. Consanguinity generally is associated with increased fertility, due at least in part to younger maternal age at marriage. Postnatal mortality is higher among consanguineous progeny, with greatest effect in the first year of life due to the expression of detrimental recessive genes. There also is evidence of greater morbidity in children born to consanguineous parents, which may extend into adulthood. With improved health care facilities, genetic disorders predictably will account for an increasing proportion of disease worldwide, and it is evident that this burden will fall disproportionately on countries and communities in which consanguinity is relatively frequent. However, it is important that the social and economic benefits of marriage to a close relative also be taken fully into consideration.