Marrese was an orthopedic surgeon who, along with another surgeon (plaintiffs), was denied membership in the American Academy of Orthopaedic Surgeons (Academy) (defendant). The plaintiffs brought suit in Illinois state court, alleging violation of their associational rights. The state court dismissed the claim. The plaintiffs then brought this action in federal court, alleging that the Academy violated federal antitrust laws. The court of appeals held that federal claim preclusion—the full faith and credit rule—barred the federal suit because Illinois antitrust law was similar to federal antitrust law. The plaintiffs appealed.