Risk factors for cancer in the general environment are studied to identify specific chemicals and classes of contaminants, to investigate mechanisms of action, and to estimate the contribution of environmental factors to cancer in the overall population. Case-control studies are conducted of non-Hodgkin's lymphoma and cancers of the bladder, colon, rectum, stomach, brain, pancreas, and kidney. The primary focus is on drinking water contaminants, especially disinfection byproducts, nitrate, and arsenic, as well as body burdens of chlorinated hydrocarbons from past environmental or dietary exposures. In the past year, two studies were completed in Western Maryland. A nested case-control study found an association between non-Hodgkin's lymphoma risk and pre-diagnostic levels of PCBs in stored serum samples. A second nested case-control study in Maryland observed a link between consumption of chlorinated surface water with elevated disinfection byproduct levels and risk of bladder cancer. A Mexican study, in which NCI researchers were major collaborators, did not find a link between breast cancer risk and serum DDT levels. Studies in Taiwan and elsewhere have described a high risk for skin, bladder, and other cancers after exposure to arsenic in drinking water supplies at levels several times the maximum contaminant limit. A case-control study in Utah is evaluating the bladder risk at lower levels that are more common the US. The possible role of nitrate in the etiology of several cancers is being evaluated in an ecologic study encompassing large areas of the United States. In addition, two case-control investigations of non-Hodgkin's lymphoma are following up on findings from another study that found a link of drinking water nitrate with this disease. Several activities are developing new approaches, and improving existing methods, of exposure asssessment in studies of general environmental exposures. These are required to better estimate risk and to detect the relatively small increases in risk often encountered in such studies. These projects often borrow on sophisticated industrial hygiene techniques developed with the Branch that are used to estimate exposure in occupational settings. Geographic information systems (GIS) are being explored for their utility in environmental epidemiology studies. Databases of water contaminants, gathered for routine monitoring purposes, are being used to estimate past exposures to subjects in case-control studies.