objectiue This article estimates lost work days and lost work day equivalents in a population sample of migraineurs, differentiating work loss due to headache episodes that met criteria for migraine from migrainous headaches not meeting full criteria and nonmigrainous headaches. Methods A random digit dialing survey of 5,071 adults identified 800 subjects with migraine headaches. By clinical examination, a subsample of 225 met migraine diagnostic criteria; 174 of these patients completed at least 11 weeks of daily diaries. This report concerns the subgroup of 122 individuals with regular paid employment. Subjects completed a daily diary over a 3-month period to assess the occurrence of headaches and International Headache Society (IHS) criteria for each headache occurrence. We report estimates of lost work days and lost work day equivalents by type of headache. Results Participants reported headaches on 8.1 work days, of which 2.2 headache days met criteria for migraine (IHS 1.1, 1.2), and an additional 2.1 headache days were migrainous without meeting full migraine criteria (IHS 1.7). On average, migraineurs missed 1.1 days of work due to headache in 3 months, of which 0.7 lost work days were due to migraine and 0.3 were due to migrainous headaches. When at work with headache, work effectiveness was reduced 41% for migraine headaches, 28% for migrainous headaches, and 24% for other headaches. Over 3 months, migraineurs experienced an average of 3.0 lost work day equivalents, of which 1.4 were due to migraine and an additional 0.7 were due to migrainous headaches. The most disabled 20% of the participants accounted for 77% of the lost work days; 40% of subjects accounted for 75% of the lost work day equivalents. Conclusions Employed migraine sufferers experienced considerable work loss and reduced work performance due to headache. The most severely affected migraineurs accounted for most of the reduced work performance. Targeting the most severely affected persons may be necessary to reduce work loss among migraineurs substantially. NEUROLOGY 1998;50:1741–1745