As is well known among financial economists but not previously recognized within the antitrust community, large and diversified institutional investors such as BlackRock, Fidelity, State Street, and Vanguard collectively own roughly two-thirds of the shares of publicly traded U.S. firms overall, up from about one-third in 1980. Recent economic research involving airlines and banking raises the possibility that overlapping ownership of horizontal rivals by diversified financial institutions facilitates anticompetitive conduct throughout the economy, and that the problem has been growing for decades, unnoticed until now. This response to an article by Professor Einer Elhauge, explains why it may be more difficult to address overlapping financial investor ownership under the antitrust laws than Elhauge recognizes.