We have recently developed an instrument to describe and categorize severity of migraine attacks from patient self-report, the MIGSEV questionnaire. We have now performed a large prospective survey using this tool to evaluate migraine severity in 2979 patients consulting for headache in France, included by 1164 general practitioners, 174 neurologists and 82 gynaecologists. The objective of the study was to determine the prevalence of severity grades in a large population who consults for migraine, to test the concordance between severity calculated from physician-derived and patient-derived data, and to test the relevance of the concept of severity as applied to diagnosis, other measures of the burden of migraine, and to health-related quality of life. Severe attacks were reported in around one-fifth of the sample. Physician- and patient-derived data provided concordant estimates of severity in 71% of cases, the discordant cases representing principally an underestimate by the physician of headache severity. Migraine severity was associated with frequent, long-lasting and treatment-resistant attacks, and with poor quality of life. The MIGSEV questionnaire is proposed as a simple measure of severity for the diagnosis and management of migraineurs, suitable for use both by physicians and patients.