The smoking practices of a national sample of Mexican-American mothers and the resulting effects of those practices on birth weight were examined. Data were from the Hispanic Health and Nutrition Examination Survey conducted by the U.S. National Center for Health Statistics during 1983-1984. We found that 24% of the mothers had smoked during their pregnancy, with a mean of 11 cigarettes per day. Infants of these women weighed 101 g less at birth than did infants of nonsmoking mothers and had a low birth weight rate of 8.0% compared with the 5.1% low birth weight rate for the sample as a whole. Multiple regression results indicate a 7.4 g decrease in birth weight for each cigarette smoked per day during pregnancy. Cultural factors that promote a low birth weight rate for Mexican-Americans that is comparable to that of non-Hispanic whites despite increased rates of poverty and inadequate health care do not protect against the insidious effects of cigarette smoking.