Ethylene oxide (ETO) is a sterilant gas considered to be a human carcinogen, due primarily to excess hematopoietic cancer in exposed cohorts. ETO causes mammary tumors in mice, and has been associated with breast cancer incidence in one small epidemiologic study. We have studied breast cancer incidence in a cohort of 7,576 women employed for at least one year and exposed for an average 10.7 years while working in commercial sterilization facilities. Breast cancer incidence (n = 319) was ascertained via interview, death certificates, cancer registries, and medical records. Interviews were obtained for 68% of the cohort. The standardized incidence ratio (SIR) for incident breast cancer in the whole cohort using external referent rates (SEER) was 0.87 (0.77, 0.97), increasing to 0.93 (0.83, 1.04) after excluding in-situ cases (6% of cases). The rate ratio for those in the top quintile of cumulative exposure, with a 15 year lag, was 1.27 (0.94, 1.69), with a positive trend (p = 0.002). Breast cancer incidence in the whole cohort was under-ascertained due to incomplete response and lack of complete coverage by state cancer registries. In internal nested case-control analyses of those with interviews, controlling for reproductive risk factors, a positive exposure-response was found with the log of cumulative exposure with a I5-year lag (p = 0.0005); the top quintile had an odds ratio of 1.87 (1.12, 3.10). Our data suggest that ETO is associated with breast cancer, but a caution is warranted due to inconsistencies in exposure-response trends and possible biases due to non-response and incomplete cancer ascertainment.