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Surnames can be used in biocultural and sociological studies to ascertain members of population groups within a city. The use of Spanish surname to ascertain Mexican ancestry has its hazards because individuals of European Spanish, Puerto Rican, Cuban, Central American or South American ancestry also have Spanish surnames. The present study questioned both parents of liveborn infants born during the first ten days of July, 1981 in the eight hospitals that offer maternity care in Denver, Colorado (N = 982). Information on fathers’ surnames, mothers’ maid­en names, parents’ birthplaces, races and ethnic groups was obtained. The association between Spanish surname and Mexican ancestry is high in the parental population. When Spanish surname is used as a predictor of partial or total Mexican ancestry, a false reading is obtained only 4.2% of the time. Only 1.6% of the individuals of Mexican ancestry are missed because they have a non-Spanish surname. Spanish surname is a less accurate predictor of 100% Mexican ancestry in the newborn population because of admixture in the parental generation.