Headache. Public health problem. Headache is, and apparently always has been, a frequent pain syndrome. It is reported in American and Western European societies in very high percentages of the population. Headache, and specifically severe headache, have also been reported as prevalent from a variety of societies worldwide, although prevalence rates have varied (they are very low, for example, in the People's Republic of China). Whether prevalence varies with different socioeconomic groups remains uncertain. Severe headache and specifically migraine is, for reasons still unknown, much more common in women, and, in most studies, is reported to decrease in prevalence in older age groups. Positive family histories are common, but the precise role of genetics is unknown. A major problem in the epidemiologic studies remains the difficulty of uniform definition of headache syndromes. 