Six passengers were injured during a severe turbulence encounter over the Pacific Ocean while the aircraft was in cruise at flight level FL330. The accident occurred in international waters near latitude 40 degrees north, longitude 152 degrees east. Review of the forecasts and pilot reports of the dispatch/weather package given to the flight crew prior to departure disclosed that an area of light to moderate turbulence was forecast along the aircraft’s planned track from 151 to 162 degrees east longitude. Pilot reports available at the time of departure consistently reported smooth conditions in the area of the forecast turbulence. At the time of the encounter the aircraft was in clear air above a cirrus cloud deck. The crew saw no convective activity ahead either visually or on radar. In the process of preparing for a position report to air traffic control, the flight crew overheard another airline flight ahead of them reporting an encounter with moderate to severe turbulence. The captain decided to secure the cabin as a precaution. The purser was notified of the decision and the captain was in the process of reaching for the seat belt sign when the turbulence was encountered. All of the injured passengers were located aft of row 50, with the majority aft of row 58. None of the injured were secured by seat belts. Probable Cause: an inadvertent encounter with unreported and unforecasted severe turbulence. A factor in the accident was the passengers’ non-use of seat belts during cruise flight.