Patent licensee estoppel would prevent the patent licensee from asserting that a patent was not valid unless the licensee breached the patent license agreement and risked a lawsuit by the patent holder for willful patent infringement. This paper discusses whether or not the U.S. Constitution requires that a patent licensee run such a risk in light of a U.S. Supreme Court decision on a drug patent in Medimmune v. Genentech. The prior positions of patent licensees, the Hatch-Waxman act, and the cases following Medimmune v Genentech are also analyzed.