This paper presents an analysis of the cost impact to the domestic petroleum refining industry in the event that effluent mixing zone dilution credits are eliminated as a result of Clean Water Act (CWA) Section 303(d) listings. Conceptual process designs and cost estimates were developed for four wastewater treatment modules at a model refinery. These modules were aimed at removing pollutants often cited as reasons for listing impaired surface waters under CWA Section 303(d). Potentially affected refineries were then identified, and compliance cost estimates were developed for these facilities by appropriately scaling results for the model facility. Costs for wastewater treatment upgrades at the individual refineries were then combined to provide an order-of-magnitude estimate of potential impacts to the industry as a whole. Of the 159 refineries operating in the United States as of late 1999, 84 were determined to discharge treated wastewater to receiving waters listed as impaired under CWA Section 303(d). From this group of potentially affected facilities, a subset of 39 refineries could require additional end-of-pipe treatment to further reduce trace concentrations of organics (principally polynuclear aromatic hydrocarbons), inorganic nitrogen (ammonia and nitrate), or cationic metals (chromium, copper, mercury, nickel and zinc) in their discharges. An additional 21 refineries could be forced to adopt a zero discharge strategy in response to stringent and technically unachievable effluent limits for selenium or, in the Great Lakes region, mercury. Estimates of compliance costs for all 60 affected refineries showed an initial capital investment totaling $1.9 billion and incremental operating and maintenance costs of $230 million per year. Industrywide, the present worth of compliance costs over a 20-year period was estimated at $4.3 billion.