A meta-analysis of published studies was conducted to identify factors which explained variation in estimates of migraine prevalence. Twenty-four population based studies contributed a total of 168 gender and age specific estimates of migraine prevalence. In linear regression analysis, 70.6% of the variation in these prevalence estimates was explained by gender, age (AGE+AGE2), a binary variable for case definition, and an interaction term between age and the case definition. Initially, we identified five groups of case definitions among the 24 studies. Only the definition of Waters (any 2 of warning, nausea, or unilateral pain) was associated with statistically significant differences in prevalence estimates among studies; accordingly the other 4 groups were combined. Several other factors were examined as predictors of migraine prevalence including the method of selecting the study population, the source of the population, the response rate and whether diagnoses were confirmed by a clinical assessment. None of these factors substantially increased explained variance. We conclude that after taking sociodemographic factors and case definition into account, estimates of migraine prevalence are remarkably stable among studies.