Abstract Background Ethylene oxide (EtO) is a carcinogenic gas used in chemical production and to sterilize medical equipment that has been linked to risk of breast and lymphohematopoietic cancers in a small number of occupational studies. We investigated the relationship between environmental EtO exposure and risk of these cancers. Methods Using the US Environmental Protection Agency’s Toxics Release Inventory, we estimated historical exposures for National Institutes of Health–AARP Diet and Health Study participants enrolled in 1995-1996. We constructed 2 metrics at 3, 5, and 10 km: 1) distance between residences and EtO-emitting facilities, weighted by the proportion of time the home was downwind of each facility, and 2) distance-weighted, wind direction–adjusted average airborne emissions index (AEI=∑lbs EtO/km2). We estimated risk (hazard ratio HR, 95% confidence interval CI) of incident breast cancer (in situ and invasive) among postmenopausal women (n = 173 670) overall and by tumor estrogen receptor status and non-Hodgkin lymphoma in the full cohort (n = 451 945). Results We observed an increased risk of breast cancer associated with EtO-emitting facilities within 10 km (HR≤10vs>10 = 1.05, 95% CI = 1.00 to 1.10) that appeared stronger for in situ (HR≤10vs>10 = 1.13, 95% CI = 1.00 to 1.27) than invasive (HR≤10vs>10 = 1.03, 95% CI = 0.97 to 1.09) disease. Risk of breast cancer in situ was also increased in the top AEI quartiles, and associations weakened with larger distances (HRQ4vs0 = 1.60, 95% CI = 0.98 to 2.61; HRQ4vs0 = 1.28, 95% CI = 0.92 to 1.79; HRQ4vs0 = 1.25, 95% CI = 1.02 to 1.53 at 3, 5, and 10 km, respectively). No differences in breast cancer risk were observed by estrogen receptor status. We found no clear pattern of increased non-Hodgkin lymphoma risk. Conclusions A novel potential association between EtO emissions and risk of in situ, but not invasive, breast cancer warrants additional evaluation.