The Supreme Court today halted an effort by a federal court to stop a class action in a state court.

The case involved claims against drug-maker Bayer over Baycol, which Bayer sold as a way to lower cholesterol. The plaintiffs alleged that Baycol killed and injured people.

The district court in Minneapolis ruled against class certification. It held that "common" questions didn't predominate, as Rule 23(b)(3) required, because each class member would have to show injury from taking Baycol and that proof of the injury would vary from class member to class member.

Bayer then asked the court to enjoin a similar lawsuit in a West Virginia state court. The federal court obliged, and the Eighth Circuit affirmed.

Reversing, the 9-0 Court concluded that the Anti-Injunction Act of 1793 barred the district court's order. The Act "broadly commands" that federal courts must leave state courts alone. Smith v. Bayer Corp., No. 09-12-5, slip op. at 5 (U.S. June 16, 2011). Bayer hadn't shown that the "relitigation exception" applied, the Court held.

It gave two reasons. Because West Virginia uses an easier test for class certification than federal courts do, Bayer couldn't meet the requirement that the state court case involve the "same issue" that the federal court ruled on. That almost the exact same rule language applied in both federal courts and West Virginia courts didn't matter. The differences in how the courts applied the test controlled.

Neither did Bayer prove that the plaintiff in the state court case counted as a "party" to the federal case. A member of a class that no court ever certified doesn't by virtue of that status become a "party" to a case that the plaintiffs hope to certify, much less a case in which the court denied certification.

The outcome likely won't affect a lot of class action cases. By passing the Class Action Fairness Act, Congress allowed removal of many potential class actions to federal court. That leaves fewer state court cases as possible targets of an anti-suit injunction.

But the Court's ruling did stress the toughness of meeting the "same issue" requirement. "Same" doesn't mean almost the same. It means the very same.

Bonus: The Court cited, for the first time, the American Law Institute's brand-new Principles of the Law of Aggregate Litigation (2010), in whose development Blawgletter played an astonishingly small role. Smith at 16 n.11.