The ultimate goal of antitrust enforcement is to maximize the surplus consumers enjoy by enhancing production efficiency and eliminating market power. Previous literature focuses on the average net wealth effects on merging firms and their stakeholder firms and reports evidence of efficiency gains while no evidence of market power in horizontal mergers. In this paper, we examine how efficiency gains distribute between the merging firms and their customer firms. We find a significant negative relation between the combined abnormal returns on the merging firms and those on their customer firms, demonstrating a wealth transfer effect. Such a negative relation is more pronounced when market power is likely to be more intensive. On average, the merging firms gain, and their customers do not lose. Our results suggest that market power allows merging firms to withhold merger gains that would have been passed to the downstream under perfect competition and prevents customers from enjoying the whole consumer surplus. Distributive inefficiency exists in horizontal mergers.