Abstract: Merger control exists to help safeguard effective competition. However, findings from a natural experiment suggest that regulatory merger control reduces the profitability of corporate acquisitions. Uncertainty about merger control decisions reduces takeover threats from foreign and very large acquirers, therefore facilitating agency-motivated deals. Valuation effects are more pronounced in countries with stronger law enforcement and in more concentrated industries. Our results suggest that competition policy may impede the efficiency of the M&A market.