Maternal age-specific rates of Down syndrome livebirths are widely utilized in personal and policy decisions concerning provision of and election of prenatal cytogenetic diagnostic services. The only extensive reference data available are on those of primarily European ancestral origin. In the absence of definitive evidence of any ethnic, racial or environmental influence upon rates (other than those associated with age) these rate schedules have been widely applied to those of all national origins. Available material age-specific data on livebirths from intensive studies on those of Hispanic (primarily of Mexican and Central American background) and of other origin in populations in the U.S.A. with likely complete ascertainment were analysed. The numbers observed were compared with (i) those predicted from established published rate schedules in those of primarily European origin, and (ii) with the observations on livebirths of non-Hispanic European origin in the same population as the Hispanic live births. In comparisons with the numbers predicted from published rates, observed numbers of case among Hispanic live births were increased by 19 per cent (SE 0.06) in younger mothers, 23 per cent in older mothers (SE 0.07) and 20 per cent (SE 0.04) in those of all ages. Comparisons with observed rates in those of Hispanic origin with those observed in non-Hispanic births in the same time intervals and populations indicated that the excess rates in Hispanics were not attributable to some local factor increasing rates in all ethnic groups at least among those under 35. Data on mothers of Mexican and Central American origin residing in the U.S.A. indicate maternal age-specific rates of Down syndrome in live births about 20 per cent greater than those in published rate schedules on Down syndrome, widely used in decisions concerning election or provison of prenatal diagnostic services. The reason for this difference remains unknown.