Although Hispanics are a poorly educated and medically underserved minority, the incidence of low birth weight (less than 2,500 g) Hispanic infants is similar to that of non-Hispanic whites. The authors used 1982-1983 Illinois vital records and 1980 US census income data to determine the contribution of maternal nativity and place of residence to this epidemiologic paradox. The proportion of low birth weight Hispanic (n = 22,892) infants ranged from 4.3% for Mexicans to 9.1% for Puerto Ricans. Maternal age, education, trimester of prenatal care initiation, and place of residence were associated with the prevalence of low birth weight infants among Puerto Rican but not foreign-born Mexican or Central-South American mothers. In very low-income (less than $10,000/year) census tracts, Mexican and other Hispanic infants with US-born mothers had low birth weight rates of 14 and 15%, respectively. In contrast, Mexican and other Hispanic infants with foreign-born mothers who resided in these areas had low birth weight rates of 3 and 7%, respectively. In a logistic model that included only impoverished infants, the adjusted odds ratio of low birth weight for those with US-born mothers equalled 6.3 (95 percent confidence interval 2.3-16.9). The authors conclude that urban poverty is negatively associated with Hispanic birth weight only when the mother is Puerto Rican or a US-born member of another subgroup.