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Retinal hemorrhage occurred in 36% of 39 subjects exposed to altitudes at or above 14,200 feet. In subjects with a history of vascular headaches at sea level, there was a higher incidence of and more severe altitude headache, as well as a higher incidence of retinal hemorrhage than among those previously headache-free. In subjects without altitude headache, none had retinal hemorrhage. In subjects with altitude headache, 42% had retinal hemorrhage. A progressive rise in the incidence of retinal hemorrhage was correlated with progressively greater intensity of altitude headache. Factors that intensified the rate or degree of exposure, including rapid ascent and strenuous exertion, appeared to increase the likelihood of hemorrhage. An optimal balance between acclimatization and subsequent altitude stress appeared to prevent retinal hemorrhage. Increased retinal blood flow, retinal vessel engorgement, increased retinal vein and prevenous capillary pressure, and possibly decreased intraocular pressure may contribute to the pathogenesis of retinal hemorrhage.