This paper uses U.S. linked birth and death records to explore associations between infant mortality and environmental factors, based on spatial relationships. The IMPLICATIONS Air pollution has been one of many suspected risk factors for SIDS, as well as for other causes of infant mortality. A previous epidemiologic study based on geographic gradients found a relationship between SIDS and PM 10 and speculated that fine particles (PM 2.5 ) might be the real culprit. However, U.S. data on SIDS clearly show a strong geographic gradient from east to west that has existed for decades, with SIDS rates in the mountain states nearly 4 times those in the big population centers of the east coast. Given that east-west air pollution patterns have changed dramatically during this period, it appears that other geographic factors should be considered. This conclusion is also supported by long-term trends in SIDS in the United States and in Scandinavia that show a dramatic increase with a subsequent decrease during a period of improving air quality.Our study considered some of those other factors, and while we confirmed a positive spatial relationship with PM 10 in 1990, we found an even stronger negative relationship with SO 4 2-aerosol, which is often a major constituent of fine particles. Thus, our study provides no support for the hypothesis that SIDS is associated with outdoor ambient concentrations of generic fine particles. Instead, the underlying implication is that epidemiologic studies based on geographic patterns can be misleading if a broad range of alternative hypotheses and candidate confounders is not also investigated. For example, we found that the variability in the association between SIDS and PM 10 due to alternative regression model specification and to the selection of locations to be considered was much larger than the variation obtained using the baseline model with different mortality end points and infant subgroups. SIDS constitutes a source of profound personal tragedy, and it is incumbent upon the research community to use the best possible science to investigate ways that SIDS risks might be reduced with some assurance of certainty.analysis considers a range of infant mortality end points, regression models, and environmental and socioeconomic variables. The basic analysis involves logistic regression modeling of individuals; the cohort comprises all infants born in the United States in 1990 for whom the required data are available from the matched birth and death records. These individual data include sex, race, month of birth, and birth weight of the infant, and personal data on the mother, including age, adequacy of prenatal care, and smoking and education in most instances. Ecological variables from Census and other sources are matched on the county of usual residence and include ambient air quality, elevation above sea level, climate, number of physicians per capita, median income, racial and ethnic distribution, unemployment, and population density. The air quality variables considere...