IN 1976 under the Safe Drinking Water Act, the U.S. Environmental Protection Agency (EPA) proposed an interim maximum contaminant level (MCL) for arsenic in drinking water of 50 micrograms per liter (μg/L) as part of the National Interim Primary Drinking Water Standards. That standard will apply until EPA adopts a new MCL. As part of a periodic review process, EPA has been reviewing data on arsenic exposure and toxicity to determine the adequacy of the 50-μg/L MCL for protecting public health. To ensure a solid and unbiased scientific basis for its arsenic standard for drinking water and surface waters, EPA requested that the National Research Council (NRC) review and comment on the arsenic toxicity data base and evaluate the scientific validity of EPA's 1988 risk assessment for arsenic in drinking water. For this report, the Subcommittee on Arsenic in Drinking Water of the NRC's Committee on Toxicology reviewed EPA's characterization of existing human health risks from ingestion of arsenic found in drinking water and food; determined the adequacy of the current EPA MCL for protecting human health in the context of stated EPA policy; and identified priorities for research to fill data gaps. The subcommittee evaluated the Taiwanese epidemiological data for carcinogenic and noncarcinogenic health effects of arsenic exposure; compared effects of arsenic exposure demonstrated in other countries, including the United States, with those documented for Taiwanese populations; and reviewed data on toxicokinetics, metabolism, and mechanism and mode of action of arsenic to ascertain how these data could assist in assessing human health risks from arsenic exposures. In areas where the subcommittee concluded that EPA could improve the toxicity analysis and risk characterization, specific changes are recommended, and the implications of the changes for EPA's current MCL for arsenic are described in this report.