On September 17, 2000, at 1820 eastern daylight time, a Bombardier CRJ, N853AS, encountered in-flight turbulence at Flight Level (FL) 260 while enroute to Atlanta Hartsfield International Airport, in Atlanta, Georgia. The airplane was operated by Atlantic Southeast Airlines as Flight 4583 under the provisions of Title 14, CFR Part 121, and instrument flight rules. Visual meteorological conditions prevailed and an instrument flight rules (IFR) flight plan was filed. The Airline Transport Captain, First Officer and 48 passengers were not injured, the Flight Attendant for the flight sustained serious injuries. The airplane did not receive any damage. The flight had originated from the Worcester Regional Airport in Worcester, Massachusetts, at 1615. According to the Captain and Flight Attendant, the flight from Worcester, Massachusetts, was smooth at FL350 until passing the Philadelphia, Pennsylvania area, at which time an area of light to moderate turbulence was encountered. The Captain instructed the Flight Attendant to take her seat and turned on the fasten seat belt sign. Flight altitude was changed three times looking for a smoother ride. The flight would shift from relatively smooth air for two or three minutes to moderate turbulence for one to two minutes. After entering an area of smooth air, the Flight Attendant continued her duties of preparing the cabin for landing. Without warning, the airplane entered another area of turbulence and the Flight Attendant was thrown about the cabin, injuring her ankles. The Captain called ahead for medical assistance to meet the airplane at the gate while the First Officer went back to check on the Flight Attendant's condition. The flight landed in Atlanta and medical personnel met the plane at the gate. Subsequent medical reports revealed that the Flight Attendant sustained a broken left ankle and a sprained right ankle. No mechanical problems with the airplane were reported by the flight crew, and weather at the time was winds 240 degrees, and 15 knots, with light to moderate turbulence.