Long QT Syndrome (LQTS) is an autosomal dominant inherited cardiac disorder associated with life-threatening arrhythmias. In northern Sweden, a LQTS founder mutation (p.Y111C, KCNQ1 gene) was verified by genetic haplotype analysis and genealogical studies, and a common ancestor couple was identified. Clinical studies of this population revealed an apparent mild phenotype. However, due to early commencement of prophylactic treatment, the natural history of this disorder cannot be properly assessed based only on clinical data. By using the family tree mortality ratio method (FTMR), we assessed the natural history of the untreated LQTS founder population. The principle of FTMR is to compare the age-specific mortality rates in a historic population harboring an inherited disorder with the corresponding mortality rates in an unaffected control population.Initially, we used the general Swedish population during the same period for comparison and observed an apparent increased longevity in the p.Y111C study population. However, when using a control population born in the same area, we observed no differences regarding overall mortality. Moreover, patterns suggesting age- and sex-stratified excess mortality, in accordance with previous LQTS studies, were evident.This study shows the importance of being aware of historical demographic patterns to avoid misinterpreting when comparing historical data.