McCollins (P) sued Bayer (D) asserting state-law claims arising from D's sale of an allegedly hazardous prescription drug called Baycol. P asked the state court to certify a class of West Virginia residents who had also purchased Baycol so that the case could proceed as a class action. Smith (P) filed state-law claims against D, similar to those raised in McCollins' suit. Smith asked the court to certify under West Virginia's Rule 23 a class of Baycol purchasers residing in the State. Neither Smith nor McCollins knew about the other's suit. D removed McCollins' case to federal court. The case was then transferred to the District of Minnesota pursuant to a preexisting order of the Judicial Panel on Multi-District Litigation, which had consolidated all federal suits involving Baycol before a single District Court Judge. D could not remove Smith's case to federal court because Smith had sued several West Virginia defendants in addition to D, and so the suit lacked complete diversity. Eventually, the District Court declined to certify McCollins' proposed class. The District Court also dismissed McCollins' claims on the merits in light of his failure to demonstrate physical injury from his use of Baycol. McCollins chose not to appeal. D then claimed that the proposed class in Smith's case was identical to the one the federal court had just rejected. D requested that the federal court enjoin the West Virginia state court from hearing Smith's motion to certify a class. The District Court agreed and granted the injunction. The Court of Appeals for the Eighth Circuit affirmed. The Supreme Court granted certiorari.