The pilot of the multiengine turboprop airplane was flying over the ocean at FL340 when he heard an “explosion.” He quickly donned his oxygen mask, declared an emergency with air traffic control, and descended to 12,000 ft. The pilot subsequently diverted the flight and landed the airplane uneventfully. After landing he observed that the airstair door had completely separated from the fuselage. The inflight separation of the airstair door and resulting depressurization of the airplane’s cabin constituted substantial damage to the airframe. Post accident examination of the airplane revealed that portions of the airstair door hinges remained attached to the fuselage, the airstair door handle anchor had failed, and the door damper support was bent and its upper bolt sheared. The six door plate strikers on the fuselage were undamaged. The airstair door was not recovered and the condition of its securing mechanisms could not be examined. Given this information, the reason for the inflight separation of the door could not be determined.