Assignment of a diagnosis of migraine has been formalized in diagnostic criteria proposed by the International Headache Society. The objective of the present study is to determine the reproductibility of the formal diagnosis of migraine in a cohort of headache sufferers over a one- year period. The study was performed in a community cohort taking part in a long- term prospective health survey, the GAZEL study. Two thousand five hundred individuals reporting headache in the GAZEL cohort were sent two postal questionnaires concerning headache symptoms and features at 12- monthly intervals. Replies to the questions allowed a migraine diagnosis to be attributed retrospectively using an algorithm based on the IHS classification scheme. The response rate was 82% for the first questionnaire and 69% for both questionnaires. Of the 1733 subjects providing information at both time- points, the agreement rate for the diagnosis of strict migraine (IHS categories 1.1 or 1.2) was 77.7% (kappa = 0.48), with 62.2% of the patients with this diagnosis (IHS categories 1.1 or 1.2) at Month 0 retaining the same diagnosis at Month 12. When diagnostic criteria were widened to include IHS category 1.7 (migrainous disorder), the agreement rate of the diagnosis was similar at 77.6% (kappa = 0.52), but 82% of the patients with this diagnosis (IHS categories 1.1 or 1.2 or 1.7) at Month 0 now retained the same diagnosis at Month 12. In conclusion, the one- year reproducibility of reporting of migraine headache symptoms is only moderate, varies between symptoms, and leads to instability in the formal assignment of a migraine headache diagnosis and to diagnostic drift between headache types. This finding is compatible with the continuum model of headache, where headache attacks can vary along a severity continuum from episodic tension- type headaches to full- blown migraine attacks. Assignment of a diagnosis of migraine has been formalized in diagnostic criteria proposed by the International Headache Society. The objective of the present study is to determine the reproductibility of the formal diagnosis of migraine in a cohort of headache sufferers over a one-year period. The study was performed in a community cohort taking part in a long- term prospective health survey, the GAZEL study. Two thousand five hundred individuals reporting headache in the GAZEL cohort were sent two post-questionnaires concerning headache symptoms and features at 12- monthly intervals. to the questions allowed a migraine diagnosis to be attributed retrospectively using an algorithm based on the IHS classification scheme. The response rate was 82% for the first questionnaire and 69% for both questionnaires. Of the 1733 subjects providing information at both time- points, the agreement rate for the diagnosis of strict migraine (IHS categories 1.1 or 1.2) was 77.7% (kappa = 0.48 ), with 62.2% of the patients with this diagnosis (IHS categories 1.1 or 1.2) at Month 0 retaining the same diagnosis at Month 12. When the diagnostic criteria were widened to include IHS category 1.7 (migrainous disorder), the agreement rate of the diagnosis was similar at 77.6% (kappa = 0.52), but 82% of the patients with this diagnosis (IHS categories 1.1 or 1.2 or 1.7) at Month 0 now retained the same diagnosis at Month 12. In conclusion, the one-year reproducibility of reporting of migraine headache symptoms is only moderate, varies between symptoms, and leads to instability in the formal assignment of a migraine headache diagnosis and to diagnostic drift between headache types. This finding is compatible with the continuum model of headache, where headache attacks can vary along a severity continuum from episodic tension-type headaches to full-blown migraine attacks.