This study was conducted to determine the level of seropositivity to diphtheria and tetanus among women in the United States, and to investigate whether the recommendation of the Advisory Committee on Immunization Practices that adolescents and adults receive a booster vaccine for diphtheria and tetanus every 10 years after primary childhood immunization is practiced by the physicians who regularly care for women. Analyses were made using data from the third National Health and Nutrition Examination Survey (NHANES III; 1988-1994). This survey provided extensive demographic information gathered from personal interviews and clinical data collected by physical examinations performed in special mobile examination centers located throughout the United States. Among the 16,123 female participants, sera samples from 9411 females 6 years of age or older were available to test for antibodies to diphtheria and tetanus. Among all participants, 57% were seropositive for diphtheria, 64% for tetanus, and 48% for both. The percentages of women immune to diphtheria decreased with increasing age so that only 27% of women 60 years old or older had diphtheria seropositivity. The same trend was seen for tetanus, with only 23% of women having immunity at 60 years of age or older, but seropositivity to tetanus remained at 80% or greater until after 39 years of age. Immunity to both diphtheria and tetanus was greatest among young girls, age 6 to 11 years. No differences in diphtheria seropositivity were seen among different racial or ethnic groups (non-Hispanic white women, non-Hispanic black women, and Mexican Americans) in the youngest age interval, 6 to 11 years, or in the group aged 50 to 59 years. The highest seropositivity to diphtheria was seen in non-Hispanic white women aged 30 to 39 years; however, by age 50, non-Hispanic white women had the lowest immunity of all raciallethnic groups. Mexican Americans were the only racial or ethnic group that had sufficient numbers to compare the seropositivity of women born in the United States with those born elsewhere. Mexican American women of childbearing age born outside the United States had significantly lower immunity to diphtheria than other racial or ethnic populations (P <0.01 for all age groups except 40-49 years, which had a P value of 0.07). These patterns were repeated for tetanus seropositivity. The lowest immunity to tetanus was seen in Mexican Americans aged 20 to 59 years who were born outside the United States (P <0.001). An age-adjusted univariate analysis was conducted of possible variables related to seropositivity for both diphtheria and tetanus, including sociodemographic and healthcare access across each racial or ethnic group. Birth outside the United States continued to be a significant risk factor for lower immunity to diphtheria and tetanus for Mexican American women (P <0.001 for both). Also, among Mexican American women, but not among other racial or ethnic groups, being at or above the poverty level and having access to regular health care were associated with higher levels of seropositivity. In general, immunity levels increased as levels of education increased, regardless of raciallethnic group. For non-Hispanic white women and Mexican American women, nulliparous women had higher levels of seropositivity than those with 2 or more live births. Multivariate modeling adjusting for age, race/ethnicity, and foreign birth found higher education to have the highest association with higher seropositivity for all women and for non-Hispanic white women. Birth outside the United States, living below the poverty level, and low levels of education were predictors of low immunity to diphtheria and tetanus among Mexican American women. There were no unique predictive factors for non-Hispanic black women.