The U.S. Supreme Court clarified the standard that federal courts must apply when determining whether to issue preliminary injunctions.   Rejecting the U.S. Court of Appeals for the Ninth Circuit’s rule that a plaintiff who shows a strong likelihood of prevailing on the merits must only demonstrate a possibility of irreparable harm, the Supreme Court reiterated that irreparable injury be “likely in the absence of an injunction.”  Winter v. Natural Resources Defense Counsel, Case No. 07-1239 (Supreme Ct., Nov. 12, 2008) (Roberts, C.J.; Breyer, J., concurring in part, dissenting in part; Ginsburg, J., dissenting),

The challenged injunction was issued by the United States District Court for the Central District of California in response to a complaint filed by the Natural Resources Defense Counsel (NRDC) that alleged the U.S. Navy’s sonar training exercises violated federal environmental laws and posed a threat to marine mammals.   The NRDC obtained an injunction requiring the Navy to shut down its sonar equipment when a marine mammal was within a fixed vicinity of a training exercise.  The district court reasoned that the preliminary injunction was appropriate because the NRDC demonstrated a “combination of probable success on the merits and the possibility of irreparable harm.”  On appeal, the Ninth Circuit affirmed, holding that the injunction was supported by “a strong likelihood of success on the merits” and “the possibility of irreparable injury to plaintiff if preliminary relief is not granted.”

The Supreme Court rejected this paradigm, declaring that “the Ninth Circuit’s ‘possibility’ standard is too lenient.”   Instead, the Supreme Court held that its “frequently reiterated standard requires plaintiffs seeking preliminary relief to demonstrate that irreparable injury is likely in the absence of an injunction.”  Granting a preliminary injunction based only on a mere possibility of harm is “inconsistent” with an injunction’s purpose as an extraordinary remedy requiring a “clear showing” of success by the plaintiff.

While the Supreme Court rebuffed the possibility standard as too soft, it did little to clarify the distinction between a “possibility” and a “likelihood” of irreparable harm.   As noted by Justice Ginsburg in her dissent, “[C]ourts do not insist that litigants uniformly show a particular, predetermined quantum of probable success or injury before awarding equitable relief.”  Instead, Ginsburg noted that courts often impose a “sliding scale” of the likelihood standard with an aim toward preserving flexibility. 

Practice Note:   Following on the heels of the Supreme Court's 2006 eBay decision, Winter will likely result in even stricter evaluation by courts of first instance when considering preliminary injunction requests in patent cases.