The relationship of headache symptoms with severity and duration of attacks. Efforts to develop clinically useful headache classification schemes have generally focused on linking specific symptom groupings with specific headache subtypes. An alternative conceptual approach, the "severity model" of headache, considers a continuum of headache ranging from mild to severe forms with specific headache subtypes distinguished by level of severity rather than unique constellations of symptoms. A population-based telephone interview was carried out among 10,169 subjects aged 12-29 to estimate the prevalence of serious headaches and better characterize symptoms that accompany headache attacks. In an analysis of frequency of occurrence, pain and duration of recent (within 4 weeks prior to interview) headache attacks, the data revealed that common symptoms (such as forehead pain and pain in the back of the head, neck and shoulders) were reported frequently, but headaches with these symptoms were generally characterized by low levels of pain and short duration. Although not an original study objective, the data were analyzed to determine whether distinct symptom constellations could be identified or whether symptoms overlapped between headache types. Symptoms of migraine were frequently experienced concomitant with tension-type symptoms; the resultant headaches were usually characterized as moderate in intensity. In contrast, symptoms usually associated with migraine in the absence of concomitant tension-type symptoms were infrequently experienced, but resulted in headaches causing the greatest disability. The data provide some support for the severity model of headache. 