Antitrust Review (Hart-Scott-Rodino)
Antitrust review under the Hart-Scott-Rodino Antitrust Improvements Act requires parties to large US mergers and acquisitions to file pre-merger notifications with the Federal Trade Commission and Department of Justice and observe a waiting period before closing, allowing regulators to evaluate whether the combination would substantially reduce competition.
The Hart-Scott-Rodino Antitrust Improvements Act of 1976 (HSR Act) established a mandatory pre-merger notification system for transactions meeting specified size thresholds. Before the HSR Act, the government often had to challenge completed mergers — unscrambling an egg — rather than preventing anticompetitive combinations before they occurred. HSR gave regulators time and information to evaluate deals before they close.
Transactions triggering HSR notification are those where both the size of the transaction and the size of the parties exceed the applicable thresholds, which are adjusted annually. As of 2025, the transaction size threshold is approximately $119.5 million. Both parties must file HSR forms with the FTC and DOJ, paying a filing fee that scales with transaction value. A 30-day initial waiting period (15 days for cash tender offers) then begins. During this period, one agency — the FTC or DOJ — reviews the filing to determine whether to open a fuller investigation.
If the reviewing agency has concerns about competitive effects, it issues a 'second request' — a broad demand for documents, data, and testimony far exceeding the initial filing. Complying with a second request can take three to six months and costs millions of dollars. The second request waiting period does not begin running until the parties have substantially complied with the document production, giving the agency broad control over the timeline. Approximately 3% of filed transactions receive second requests in a typical year.
After reviewing the second request materials, the agency may clear the deal with no conditions, negotiate remedies — typically divestitures of overlapping business lines — or seek a preliminary injunction to block the transaction entirely. Major enforcement actions in recent years, including DOJ challenges to AT&T/Time Warner and FTC challenges to Meta/Within, have demonstrated that even well-funded and legally sophisticated deal teams face genuine regulatory risk in concentrated markets.
HSR compliance is one of the primary closing conditions in US merger agreements. Deals are typically conditioned on receiving HSR clearance, and the parties must cooperate in good faith to satisfy regulatory requirements — while balancing the need to preserve their operating independence until closing.