THE literature on the welfare costs of monopoly and the estimated consumer gains from its elimination' provides at least an implicit basis for formulating antitrust policy in cost-benefit terms. In principle antitrust agencies can use welfare loss models to measure the benefits of bringing particular cases and other actions against firms. Then weighing these benefits against the associated costs of bringing antitrust actions, the agencies can determine their optimal resource allocation strategy. But while easily formulated, what do these normative prescriptions for antitrust policy have to do with the actual behavior of antitrust agencies? And what role (if any) do economic measures of the costs and pervasiveness of monopoly power play in the conduct of the affairs of the government overseers of competition? This study derives and tests linkages between the prescriptions of economists and the actual casebringing activity of the Antitrust Division of the Justice Department over the last twenty-five years. We draw on the recently published data of Posner2 on how antitrust activities have in fact been conducted in the past and compare a benefit-cost model of desirable antitrust behavior with actual antitrust activity to assess the role of economic factors in the conduct of antitrust policy. In effect we draw on the prescriptive studies of economics to see to what extent industry welfare losses, and the components of welfare losses which measure the price impact of monopoly and the industry size, explain the historical distribution of antitrust cases across different manufacturing industries.