In 1997, when Pall Corp. filed a patent infringement lawsuit against what's now a unit of The 3M Co., neither side could have expected that the case would still be kicking around two decades later. But after churning through six law firms, seven judges, and more than 17 years of litigation, the case is still alive—and now it may finally be headed to trial.

In a decision issued Tuesday, U.S. District Judge Pamela Chen in Uniondale, New York, ruled that 3M Purification Inc. can't escape Pall's infringement claims, which relate to a now-expired patent for water filtration and purification technology.

The litigation—possibly the oldest active patent case in the country—has been drawn out by a series of failed injunction bids and by three separate requests from 3M and its predecessor Cuno Inc. for reexaminations of Pall's patent by the U.S. Patent and Trademark Office. As it stands now, the case includes some claims left over from Pall's initial 1997 complaint, along with claims from a 2003 complaint that took aim at later-developed Cuno products.