In an epidemiologic study at high altitude and sea level, we found a 12.4% prevalence rate of migraine in the high-altitude and 3.6% in the sea-level populations. In both locations, women had a greater frequency of headaches, although the differences were less at high altitude due to a high frequency of migraine among men who were miners. Possible aggravating factors among miners include working extra hours or night shifts in the mines. © 1991 American Academy of Neurology.