Distribution of consanguineous, marriages among spouses, husband's parents and wife's parents was analyzed based upon data on 20,563 families in six populations in Japan. There were highly significant correlations of presence or absence of consanguinity among the three couples in most populations. That is, products of consanguineous marriages tend to marry each other and a product of consanguineous marriage tend to marry a relative. Such familial accumulation of consanguineous marriages may lead to a higher inbreeding level in the offspring of consanguineous unions than that estimated from parental relationship only. Moreover, maternal inbreeding effects may overlap the consanguinity effects. Therefore, the effects of consanguinity may be overestimated, if familial accumulation of consanguineous marriages is ignored.