A randomly selected sample of 1668 individuals (782 women and 886 men) aged 18-74 years was interviewed by telephone using a standardized questionnaire including the International Headache Society (IHS) criteria for migraine. The survey was performed by SIFO, the national public opinion poll agency. The results obtained demonstrate that 21% of the Swedish population had suffered from severe headaches during the past year. A majority of these headache sufferers (61%) fulfilled the IHS-criteria for migraine. The 1-year prevalence of migraine in Sweden was found to be 13.2 +/- 1.9% (16.7% among women and 9.5% among men). The prevalence of migraine in this Swedish population did not differ between the northern, middle and southern part of Sweden, or between urban and rural areas or different income groups. Only about half (49%) of the migraineurs had been diagnosed by a physician. Among the individuals who fulfilled the IHS criteria for migraine the mean attack-frequency was 1.3 per month and the mean attack-duration was 19 h. If the duration of the attack was disregarded as a strict criterion for migraine (attacks < 4 h and > 72 h were included), the prevalence increased to 17.0 +/- 1.9% without affecting the sex distribution. With this amendment, 92% of those who considered their headaches to be migraine or migraine-like in fact fulfilled the alternative criteria for migraine. However, only 76% of those who believed that they had migraine or migraine-like headaches fulfilled the strict IHS criteria for migraine. An extension of the time window from 4 to 72 h may be reasonable both from a pragmatic and from a rational clinical point of view.