"November, 2003." Although studies suggest that air pollution is linked to perinatal outcomes, the geographic characterization of exposure to pollution differs between the studies. Thus, we compared neighborhood and county-level measures of air pollution exposure, while examining the association between particulate matter less than 2.5 micrometers in aerodynamic diameter (PM2.5) and birth weight among full-term births in California in 2000. Our analysis was limited to two populations of 8,579 non-Hispanic white and 8,114 Hispanic mothers who were married, between 20 and 30 years of age, completed at least a high school education, and gave birth for the first time to reduce the effects of demographic variability. Measurements from the nearest monitor, average and distance-weighted average of monitors within a five-mile radius from each mothers residence (defined as neighborhood metrics) and the mean of monitors within each mothers county of residence were considered. PM2.5 measurements, provided by the California Air Resources Board, were calculated to correspond to each mothers nine-month gestation period. Although metrics within the five-mile radii and the county were highly correlated (r2 = 0.78), the county-level metric provided a stronger association between PM2.5 and birth weight (beta = -4.04, 95% confidence interval = -6.71, -1.37) than the metric for the average of all monitors within five-miles (beta = -1.38, 95% confidence interval = -3.36, 0.60) among non-Hispanic white mothers; similar results were observed among the Hispanic sample of mothers. Consequently, inferences from studies using different definitions of air pollution exposure may not be comparable. Includes bibliographical references.