The Agency for Toxic Substances and Disease Registry has, as mandated in Superfund legislation, established the National Exposure Registry (NER). The purpose of the NER is to assess and evaluate the potential relationship between adverse health effects and environmental exposure for an exposed population, particularly the relationship between chronic health effects and long-term, low-level chemical exposures. The NER's primary goal is to facilitate epidemiology research by establishing multiple data bases (subregistries) that contain demographic, environmental, and health information on large populations exposed to selected chemicals. The Registry data mainly serve the purpose of being hypothesis-generating rather than hypothesis-testing. The NER is currently composed of subregistries of: (1) persons exposed to volatile organic compounds (VOCs)--a subset of registrants in whom trichloroethylene (TCE) is the primary VOC exposure, but others are present (N = 4,832), a subset in whom benzene is the primary VOC exposure (N = 1,142), and a subset in whom trichloroethane (TCA) and TCE are the highest VOC exposures (N = 3,666); and (2) persons with dioxin exposure (N = 250). Chromium and radioactive substances subregistries are planned.