This study examined the association between particulate matter < or =10 microm in aerodynamic size (PM(10)), carbon monoxide (CO), and ozone (O(3)), and birth weight in Washoe County, NV, from 1991 through 1999. In total, 39,338 single births were included in this study. The mean birth weight was 3383 +/- 460 g and prevalence of low birth weight (LBW) was 2.46% for single births with a gestational age of 37-44 wk. After controlling for cofactors including infant sex, maternal residential city, education, medical risk factors, active tobacco use, drug use, alcohol use, prenatal care, mother's age, race and ethnicity of mothers, and weight gain of mothers, we found PM(10) exposure in the third trimester of pregnancy to be a significant predictor of birth weight of newborns. A 10-microg/m(3) increase in the 24-h PM(10) level in the third trimester of pregnancy can be associated with a birth-weight reduction of 11 g (95% CI: 2.3-19.8 g) using multiple linear regression; however, PM(10) was not found to be related with the risk of LBW from logistic regression. CO and O(3) were not found to be associated with birth weight or risk of LBW of newborns by the same modeling procedure. The results suggest PM(10) could be a risk factor associated with birth-weight reduction of newborns in urban northern Nevada; however, the current level of PM(10) is not a risk factor to increase the chance of LBW newborns.