Flight helmets have been recommended as aircrew head protection since 1908, yet debate continues regarding their effectiveness. Estimates of helmet use in civilian helicopter aeromedical programs range from 6.5% to 13%. The effectiveness of the Army's SPH-4 flight helmet in reducing severe head injuries sustained during helicopter accidents was evaluated using the accident data base at the US Army Safety Center, Fort Bucker, Alabama. Analysis was restricted to severe (Class A) accidents that were at least partially survivable, using US Army Safety Center criteria. Occupants not wearing a protective helmet were significantly more likely to sustain severe and fatal head injuries than were occupants wearing the SPH-4 (BR = 3.8 and 6.3, respectively; P < .01). Unhelmeted noncockpit occupants were at higher risk of head injuries (BB = 5.3 and 7.5; P < .01). All personnel regularly participating in helicopter flight, civilian or military, should be equipped with protective headgear.