Despite numerous studies of air pollution and adverse birth outcomes, few studies have investigated preeclampsia and gestational hypertension, two pregnancy disorders with serious consequences for both mother and infant. Relying on hospital birth records, we conducted a cohort study identifying 34,705 singleton births delivered at Magee-Women's Hospital in Pittsburgh, PA between 1997 and 2002. Particle (<10 μm-PM10; <2.5 μm-PM2.5) and ozone (O3) exposure concentrations in the first trimester of pregnancy were estimated using the space-time ordinary Kriging interpolation method. We employed multiple logistic regression estimate associations between first trimester exposures and preeclampsia, gestational hypertension, preterm delivery, and small for gestational age (SGA) infants. PM2.5 and O3 exposures were associated with preeclampsia (adjusted OR = 1.15, 95% CI = 0.96-1.39 per 4.0 μg/m(3) increase in PM2.5; adjusted OR = 1.12, 95% CI = 0.89-1.42 per 16.8 ppb increase in O3), gestational hypertension (for PM2.5 OR = 1.11, 95 % CI = 1.00-1.23; for O3 OR = 1.12, 95 % CI = 0.97-1.29), and preterm delivery (for PM2.5 ORs = 1.10, 95% CI = 1.01-1.20; for O3 ORs = 1.23, 95% CI = 1.01-1.50). Smaller 5-8 % increases in risk were also observed for PM10 with gestational hypertension and SGA, but not preeclampsia. Our data suggest that first trimester exposure to particles, mostly PM2.5, and ozone, may increase the risk of developing preeclampsia and gestational hypertension, as well as preterm delivery and SGA.