The airplane was at flight level 350 with the autopilot engaged, and about 35 to 40 minutes from its destination, when it encountered severe turbulence. The captain had turned the seat belt light on 5 to 10 minutes before the turbulence, and the first officer had made the public address announcing that everyone needed to remain seated with their seatbelts fastened. Suddenly, the airplane encountered severe turbulence and climbed rapidly to about 500 feet above the assigned altitude; the autopilot then disengaged. The captain took control of the airplane and slowly lowered the nose. As the captain leveled the airplane, the nose pitched down and the airplane descended about 500 feet below the assigned altitude before leveling and departing the area of turbulence. At the time of the encounter, the airplane was in a layer of haze. There was no heavy precipitation noted on the radar, and no build-ups visible to the flight crew. One passenger was seriously injured, and two passengers and two flight attendants sustained minor injuries. Probable Cause: An unanticipated inflight encounter with turbulence.