To determine independent effects of maternal smoking and infant low birth weight (less than 2,500 g) on risk of sudden infant death syndrome (SIDS) among different ethnic groups, the authors conducted a population-based case-control study based on the 1984-1989 Washington State birth record data. Two control groups were selected for 916 SIDS cases. The first one comprised 3,704 randomly selected controls, matched to cases by birth year, to describe the characteristics of the study population. In the second control group (n = 6,186), minorities were oversampled, by matching to cases on maternal race/ethnicity and birth year, to increase the power of analysis within each ethnic group. All subjects were classified into five groups on the basis of maternal race/ethnicity: white, black, American Indian, Asian, and Hispanic. After controlling for confounders, the authors found that maternal smoking was independently associated with SIDS among white (odds ratio (OR) = 2.2, 95% confidence interval (CI) 1.8-2.6), blacks (OR = 3.1, 95% CI 1.7-5.9), Asians (OR = 2.7, 95% CI 1.1-6.6, and Hispanics (OR = 5.5, 95% CI 1.4-22.0), but had little relation among American Indians (OR = 1.4, 95% Cl 0.9-2.4). Infant low birth weight was independently related to SIDS among whites (OR = 2.5, 95% Cl 1.8-3.4) and American Indians (OR = 5.5, 95% Cl 2.8-11.2) and to a lesser extent among blacks (OR = 1.9, 95% Cl 0.8-4.1), but not among Asians (OR = 1.1, 95% Cl 0.2-5.2) or Hispanics (OR = 1.2, 95% Cl 0.1-11.5). The misclassification that may occur because of the application of the same definition of low birth weight to all ethnic groups may be the main reason for the weaker association between infant low birth weight and SIDS among blacks and the absence of an association among Asians and Hispanics. Defining low birth weight as below population mean minus 1.96 standard deviations may provide better insight into the relation between low birth weight and SIDS. Understanding the reasons for the lack of a strong association between maternal smoking during pregnancy and SIDS among American Indians may enhance our knowledge of the etiology and pathogenesis of SIDS.