On February 7,1999, at 0827 mountain standard time, a Boeing 757-222, N586UA, was not damaged following an encounter with turbulence near Climax, Colorado. The 2 airline transport pilots, 5 flight attendants, and 22 passengers were not injured; however, 1 flight attendant was seriously injured. The airplane was being operated by United Air Lines, Inc., under Title 14 CFR Part 121. Visual meteorological conditions prevailed for this scheduled domestic passenger flight which originated from Denver, Colorado, approximately 30 minutes before the accident. An IFR flight plan had been filed with a destination of Santa Ana, California. FAA records indicate that air traffic control cleared United Airlines flight 553 to an altitude of 35,000 feet via the Rockies Three Departure (DEN R-240). At approximately 0820, the captain of flight 553 radioed ahead to another United flight on the same departure course and altitude, requesting weather status. The other United flight reported "occasional light chop, uh, mostly smooth." The captain released the flight attendants to move about the cabin. The captain reported that between FL340 and FL345, the flight encountered severe turbulence. One flight attendant received a fractured knee cap and a concussion. United Airlines records indicate that the flight crew had been notified of a Significant Meteorological (SIGMET) report for moderate to occasionally severe turbulence between FL300 and FL420. An FAA inspector with the Denver FSDO determined that United flight 553 was south of the SIGMET box at the time of the severe turbulence event.