Mortality is one of the most widely used indicators in small-area ecologic studies. Both accessibility to mortality data and advances in the development of new disease mapping techniques have contributed to an abundance of mortality maps and atlases over the last decade. Results may be biased in this kind of study if there has been unmeasured geographic mobility of the population. Most published papers tend to neglect this possibility. We use the theory of dynamics systems to demonstrate that migratory flows unmonitored by official population registers may lead to major errors in mortality rates and relative risks. Simulations in 4 scenarios showed more than 8% underestimation of the mortality rate and more than 11% underestimation of relative risk in areas with high uncontrolled emigration, and above 19% overestimation of mortality rate and above 15% overestimation of relative risk in areas with high uncontrolled immigration.Researchers conducting small-area epidemiologic studies should explore the reliability of population information in geographic areas before drawing hypothesis or conclusions on other possible causes of mortality differences.