The Ninth Circuit Court of Appeals for many years has used a "sliding scale" test to evaluate whether a preliminary injunction is proper. It looks at the usual issues--likelihood of success on the merits and threat of irreparable harm--but says that strong evidence of likely success means the threat of irreparable harm need only be a "possibility," not a likelihood.

The U.S. Supreme Court has now decided an environmental case that is significant in disapproving the "possibility" standard. The high Court requires that the threatened irreparable harm be likely regardless of how strong the plaintiff's showing on the merits of the suit.

Although decided in the context of an environmental case, the opinion will be applied to any other cases in which preliminary injunctive relief is sought by a plaintiff at the outset of litigation and pending a final adjudication on the merits. For example, intellectual property cases seeking to enforce copyrights, trademarks, trade names, or patents will be subject to the requisite standard for proving evidence of irreparable harm. Suits alleging unfair competition under the Lanham Act also will have to show probable irreparable harm. In IP cases, however, there are certain presumptions that at times can come into play to assist the showing of threatened harm.

The Supreme Court case, decided on November 12, is Winter v. Natural Res. Defense Council, Inc., No. 07-1239