This article discusses how decreasing the existing arsenic maximum contaminant level (MCL) for drinking water will impose substantial compliance costs on water suppliers in the United States. A rigorous methodology was developed to estimate the national cost of complying with alternative regulatory limits for arsenic. This methodology considered the feasibility of available technologies based on existing treatment at utilities that are not in compliance and the level of water quality constituents that limit technology performance. A least-cost method of selecting treatment alternatives was used to estimate compliance costs, and the results were extrapolated nationally. Includes 35 references, tables, figures.