Michael Nelson for Venable, LLP reports, at least in some circumstances, the USPTO can find a patent invalid, even after a court has upheld its validity.
Baxter International, Inc. owns a patent related to hemodialysis machines. In 2003, Fresenius Medical Care Holdings filed for declaratory judgment of non-infringement and invalidity. In 2005, Fresenius also requested reexamination of the patent in suit. In 2007, the district court agreed with Baxter and granted judgment of validity and infringement (affirmed on appeal in 2009). Between 2006 and 2010, the USPTO reexamined the patent. The Board of Appeals at the USPTO, even after consideration of the Federal Circuit decision, found the claims obvious, invalidating the patent (the Federal Circuit upheld the Board of Appeals in In re Baxter).
Please see full article below for more information.