On March 1, 2003, at 0630 eastern standard time, an Aerospatiale ATR-72-212, N642AS, registered to and operated by Atlantic Southeast Airlines as Flight 4268, encountered turbulence during cruise flight. Flight 4268 was operated under the provisions of Title 14 CFR Part 121, as a scheduled, domestic passenger flight from Valparaiso, Florida, to William B Hartsfield Atlanta International, Atlanta, Georgia. The flight operated under instrument flight rules with a flight plan filed. The airline transport-rated pilot in command, airline transport-rated first officer, and 12 revenue passengers on board were not injured. One cabin attendant received serious injuries, and the other cabin attendant received minor injuries. The flight continued to Atlanta, Georgia, without further incident. The flight departed Eglin Air Force Base, Florida, at 0450 central daylight time on March 1, 2003. According to the captain, during cruise flight at 17,000 feet the airplane encountered a jolt of severe turbulence. The autopilot disconnected and there was a 300-foot loss in altitude. According to the flight attendants, during the turbulence encounter they were pinned to the ceiling and dropped to the floor. The captain called to check on the flight attendants status. The flight attendants informed the captain the passengers were uninjured and they had received minor injuries. At the time of the turbulence encounter the seat belt sign was illuminated. The captain radioed for paramedic support after landing. After landing the paramedics boarded the airplane, examined the attendants, and advised them to go to the emergency room for further evaluation. At the emergency room it was determined that one fight attendant had a broken femur in the left ankle and the other was not injured. The nearest weather reporting facility at the time of the accident was Atlanta International, Georgia. The 0553 surface weather observation was: 400 overcast, visibility 6 statue miles, temperature 7 degrees Celsius, dew point 6 degrees Celsius, wind 70-degrees at 11 knots, and altimeter 30.06 inches. According to the operator, the flight crew received a weather briefing before the flight departed, however there was no weather warning given for severe turbulence.