The Ninth Circuit Court of Appeals has refused to revisit its June ruling reviving antitrust claims against CoStar, leaving the real estate giant facing accusations it has monopolized commercial listing markets.

On Friday, the appellate panel denied CoStar’s petition for rehearing, a last-ditch effort to unwind the earlier decision that gave rival Commercial Real Estate Exchange Inc. (CREXi) new momentum in its lawsuit. Judges Lucy Koh and Anthony Johnstone voted against rehearing, and no other judge called for a full panel review.

“The Ninth Circuit has now definitively rejected CoStar’s appeal, ruled in favor of CREXi, and green lit CREXi’s antitrust claims,” said Nicholas S. Goldberg of Keker Van Nest & Peters LLP, counsel for CREXi. “CoStar’s efforts to bob and weave, and delay its day of reckoning for its monopolistic and oppressive behavior, have run their course.”

CoStar Pushes Back

CoStar’s general counsel Gene Boxer countered Monday that the company took solace in the court amending parts of its June opinion. The new version, he argued, requires CREXi to prove that nearly every CoStar client misunderstood its terms.

“CREXi’s makeweight counterclaims never made any sense, and they make even less sense now,” Boxer said. He also noted that many brokers maintain listings with both CoStar and CREXi, undercutting the exclusivity allegations.

Meanwhile, CoStar continues to press its copyright lawsuit against CREXi, accusing it of copying photographs and data. CREXi has sought to pause that case until its antitrust claims are resolved.

Monopoly Allegations Revived

The Ninth Circuit’s June decision marked a sharp reversal of a 2023 California district court ruling that tossed CREXi’s claims. The lower court had found insufficient evidence of monopoly power and ruled CoStar’s broker contracts were not exclusive.

But the appellate panel saw it differently. CREXi alleged CoStar’s contracts effectively locked in brokers, while technological barriers created a moat around its platform. The panel highlighted evidence that CoStar raised prices after acquiring competitors or forcing rivals from the market—classic markers of monopoly power.

Broader Antitrust Stakes

The battle has drawn heavyweight interest. In August, a tech industry group and antitrust scholars sided with CoStar, warning the Ninth Circuit’s stance could upend long-standing antitrust doctrine.

The Chamber of Progress argued the ruling might pressure companies to share proprietary systems with rivals, “sidestepping foundational refusal-to-deal principles” and potentially chilling innovation across technology-driven markets.

Counsel in the Fight

• CoStar: Elyse M. Greenwald, Melissa A. Sherry, Nicholas J. Boyle, Jordan R. Goldberg, and Jeremy L. Brown of Latham & Watkins LLP.

• CREXi: Elliot R. Peters, Dan E. Jackson, Warren A. Braunig, and Nicholas S. Goldberg of Keker Van Nest & Peters LLP.

• Amici: Chamber of Progress, represented by Kathleen E. Farley in-house, and Steven P. Lehotsky and Joshua P. Morrow of Lehotsky Keller Cohn LLP; antitrust scholars represented by Cody L. Reaves and Peter L. Hamilton of Torridon Law PLLC and others from Sidley Austin LLP.