Most assessments of the vigor of antitrust enforcement rely on statistics as to the numbers of cases that are initiated: More cases imply more vigor. But if deterrence is a factor in the enforcement process, then there ought to be an inverted parabola – a “Laffer curve” – that represents the relationship between cases brought and enforcement efforts. And on the right-hand-side of this curve, more vigorous enforcement effort is associated with fewer cases that are brought: because there are fewer violations to be detected and prosecuted. The effects of deterrence need to be integrated into assessments of enforcement.