Endometriosis is a gynecologic disorder defined as the presence of endometrial tissue outside the uterus and is associated with pelvic pain and infertility. Up to 10% of U.S. women of reproductive age may be affected by this disease. It has been hypothesized that environmental contaminants such as polychlorinated biphenyls (PCBs) contribute to endometriosis risk by affecting steroid hormones. Exposure to dioxin-like “coplanar” PCBs, which induce biologic effects through binding to the aryl hydrocarbon receptor, is generally not associated with endometriosis. Studies exploring the potential for “non-coplanar” PCBs, which have no or only weak dioxin-like toxicity, to produce endometriosis have been inconsistent. In a case–control study of Group Health Cooperative enrollees in western Washington State, Trabert et al. (p. 1280) measured 20 PCB congeners in serum from surgically confirmed endometriosis cases newly diagnosed between 1996 and 2001 and from female controls matched for age and reference year. The authors conclude that non-coplanar PCB concentrations at levels measured in women from western Washington State do not contribute meaningfully to endometriosis risk.