On March 23, 1996, at 0920 mountain standard time, a United Air Lines Boeing 757-222, N503UA, encountered unforcasted clear air turbulence in the vicinity of Taos, New Mexico, while in cruise flight at flight level 330. The crew of two pilots and five flight attendants were not injured, and 1 of the 134 passengers received serious injuries. Visual meteorological conditions prevailed for this Title 14 CFR Part 121 scheduled air carrier flight en route from Orange County Airport, Santa Ana, California, to O'Hare Airport, Chicago, Illinois, and an IFR flight plan was filed. The flight had been airborne approximately one hour and 30 minutes. According to the crew, the flight encountered unforecasted and unexpected clear air turbulence. The seat belt sign was not illuminated, and an elderly passenger, who was on her way to the lavatory, fell and struck her arm on an arm rest. She received initial treatment from a physician who was on the flight and the flight was met by paramedics upon arrival. They determined the passenger had broken the humerus in her right arm. At the time the accident occurred, there were no pilot reports or other informational notices regarding turbulence.