This analysis examines incidence rates, prevalences, and outpatient encounters for migraine and other headache syndromes among active component members of the U.S. Armed Forces from 1998 through 2010. For both migraine and other headache syndromes, incidence rates, prevalences, and rates of outpatient encounters increased during the period. In 2010, 3.9 percent of male service members and 11.3 percent of females had at least one outpatient encounter for an episode of headache; rates were higher among females than males. Among service members ever diagnosed with migraine, 3 percent of men and 6 percent of women had more than 10 encounters for migraine; for other headache syndromes, the respective percentages were less than 1 percent. The introduction of new ICD-9 codes during the period had little effect on the coding practices for migraine, but did modestly affect the coding practices for other headache syndromes.