Washington State reports one of the highest rates of sudden infant death syndrome (SIDS) in the United States; within the state, Native Americans have the highest rate of any racial group. To explore this apparent genetic predisposition, we conducted a population-based retrospective cohort study. Using the state's linked birth and death certificate file for 1984 to 1988, we compared infants whose mothers were coded as "American Indian" with infants whose mothers were coded as "white." Native American infants were more than three times more likely than white infants to die of SIDS (crude relative risk = 3.25; 95% confidence interval = 2.41 to 4.38). However, this elevated risk diminished after adjustment for differences between Native American and white mothers in age, marital status, parity, and smoking status during pregnancy (adjusted relative risk = 1.82; 95% confidence interval = 1.28 to 2.58). The high SIDS rate of Washington's Native Americans appears to be due to the high prevalence of SIDS risk factors among Native American mothers, rather than to a genetic predisposition in the infants. Because many of these maternal factors are related to socioeconomic status, it is likely that programs to improve the overall health of Native Americans might lessen both the impact of SIDS and that of other causes of infant morbidity and death.