Ethylene oxide is a gas used in some dental offices to sterilize equipment. In pregnant laboratory animals, ethylene oxide increases malformations and fetal loss. Increased gestation length has also been reported. In humans, two studies have reported increased spontaneous abortions among ethylene oxide-exposed women, but few other data exist. We sent questionnaires to 7,000 dental assistants, age 18-39 years, registered in California in 1987; 4,856 responded (69%). We based our analysis on 1,320 women whose most recent pregnancy was conceived while working full-time. Thirty-two women reported exposure to ethylene oxide; unexposed dental assistants comprised the comparison group. We estimated relative risks of spontaneous abortion and preterm birth using a person-week model. We estimated relative risks of postterm birth (≥42 weeks) and a combined adverse outcomes model using logistic regression. Among exposed women, the age-adjusted relative risk of spontaneous abortion was 2.5 [95% confidence interval (CI) = 1.0-6.3], for preterm birth 2.7 (95% CI = 0.8-8.8), and for postterm birth 2.1 (95% CI = 0.7-5.9). The estimated relative risk of any of these adverse outcomes among exposed women was 2.5 (95% CI = 1.0-6.1) after adjusting for age, nitrous oxide, and number of mercury amalgams prepared. These data further implicate ethylene oxide as a possible reproductive toxicant in humans.