The Migraine Disability Assessment (MIDAS) questionnaire is a brief, self-administered questionnaire designed to quantify headache-related disability over a 3 month period. The MIDAS score has been shown to have moderately high test-retest reliability in headache sufferers and is correlated with clinical judgment regarding the need for medical care. The aim of the study was to examine the validity of the MIDAS score, and the five items comprising the score, compared to data from a 90 day daily diary used, in part, to record acute disability from headache. In a population-based sample, 144 clinically diagnosed migraine headache sufferers were enrolled in a 90 day diary study and completed the MIDAS questionnaire at the end of the study. The daily diary was used to record detailed information on headache features as well as activity limitations in work, household chores, and non-work activities (social, family and leisure activities). The MIDAS score was the sum of missed work or school days, missed household chores days, missed non-work activity days, and days at work or school plus days of household chores where productivity was reduced by half or more in the last 3 months. Validity was assessed by comparing MIDAS items and the MIDAS score with equivalent measures derived from the diary. The MIDAS items for missed days of work or school (mean 0.96, median 0) and for missed days of household work (mean 3.64, median 2.0) were similar to the corresponding diary-based estimates of missed work or school (mean 1.23, median 0) and of missed household work (mean 3.93, median 2.01). Values for missed days of non-work activities (MIDAS mean 2.6 and median 1 versus diary mean 2.22 and median 0.95) were also similar. Responses to MIDAS questions about number of days where productivity was reduced by half or more in work (mean 3.77, median 2.00) and in household work (mean 3.92, median 2.00) significantly overestimated the corresponding diary-based measures for work (mean 2.94, median 1.06) and household work (mean 2.22, median 0.98). Nonetheless, the overall MIDAS score (mean 14.53, median 9.0) was not significantly different form the reference diary-based measure (mean 13.5, median 8.4). The correlation between the MIDAS summary score and an equivalent diary score was 0.63. The group estimate of the MIDAS score was found to be a valid estimate of a rigorous diary-based measure of disability. The mean and median values for the MIDAS score in a population-based sample of migraine cases were similar to equivalent diary measures. The correlation between the two measures was in the low moderate range, but expected given that two very different methods of data collection were compared.