On June 11, 1999, at 2118 central daylight time (cdt), a Boeing 777-222, N781UA, operated by United Airlines as flight 256 encountered turbulence during a descent near Madison, Wisconsin. Four passengers received minor injuries and one passenger was seriously injured. Four flight attendants received minor injuries. Both cockpit crewmembers, 6 flight attendants, and 279 passengers were not injured. The airplane was not damaged. The 14 CFR Part 121 passenger flight was operating in visual meteorological conditions on an IFR flight plan. The flight was originally scheduled from Denver, Colorado, to Chicago, Illinois; however, it diverted to Minneapolis, Minnesota due to thunderstorms in the Chicago area. The flight departed from Minneapolis, Minnesota, at 2048 cdt. The flight continued on to Chicago after the turbulence encounter where it landed uneventfully, at the Chicago O'Hare International Airport, at 2149 cdt. The captain reported the flight was smooth up until the unexpected turbulence was encountered while they were descending through 18,000 feet msl. He reported the moderate turbulence lasted 20 to 30 seconds after which the remainder of the flight was smooth. The seat belt sign was not on at the time of the turbulence encounter. The captain reported the radar was on and it was showing no indication of cells or turbulence along the route. He reported there was a rain shower at their 2 o'clock position, 30 miles away. The first officer reported that moderate turbulence was encountered during the descent at an altitude of about 17,000 feet msl. He reported, "It consisted of a 2-3 second period of negative "g" followed by a 2-3 second positive "g" deviation, then a two to five minute period of light to moderate slight deviations from our flight path." He reported there was nothing on the radar in their area, but convective activity was displayed beyond the Chicago area. The flight attendants reported that all the passengers that were injured were out of their seats and in the back of the airplane when the turbulence was encountered. The most serious injury was a passenger who suffered a broken ankle. According to United Airlines, the flight crew received a Weather Briefing Message (WBM) prior to departing Minneapolis, which contained convective sigmets for Wisconsin, Illinois, and Indiana. The accident took place ahead of a stationary front which extended northeast-southwest through central Wisconsin and Iowa which had been creating shower and thunderstorm activity throughout afternoon and evening on the day of the accident. A review of recorded aircraft data revealed the flight experienced a change in G-loading from .0843 g's to 1.9375 g's over a 2-second time period while at an altitude of 19,680 feet msl.