Previous research has shown that the persistent regional mortality differences in Finland cannot be attributed to sociodemographic factors, health behaviors, or living conditions at young age. Using longitudinal population register data from Finland, we analyze the effect of birth region and ethnicity on mortality risks in men age 65–74 years and women age 75–84 years. Mortality from ischemic heart disease in particular is found to be highly influenced by these proxies for people's ancestry. The results are consistent with findings from population genetic and medical research, which says that genetic diversity is great across major regions of Finland and that this diversity may underlie variation in disease susceptibility. We argue that the regional mortality variation might reflect geographic clustering of hereditary factors, enforced by Finland's specific population development. We illustrate that large-scale population register data without genetic information or biomarkers can be useful for understanding cause-specific mortality within a national population.