On June 24, 1993 at 1229 central daylight time, a Boeing 757-222, N579UA, operated by United Airlines, Inc., as flight number 1218, encountered moderate turbulence in the clouds at flight level 290, over Baraboo, Wisconsin, while descending on a vector for an approach runway 14R at Chicago O'Hare International Airport. The operator reported the seat belt sign was on, the weather radar was on, and the airplane was clear of any green returns on the radar. One flight attendant sustained serious injuries. There were no injuries to the two cockpit crewmembers, five other flight attendants, or 140 passengers aboard the airplane. The flight was operating under 14 CFR Part 121, and originated at Vancouver, British Columbia, Canada, at 0920. The destination was O'Hare Airport. An IFR flight plan was filed, and instrument meteorological conditions prevailed at the time. During the turbulence encounter, the flight attendant fell and injured her left ankle. She was treated for the injury on completion of the flight. The injury was diagnosed as a fractured ankle.