We studied the impact of the Finnish-enriched SCN5A missense variant (rs45620037 [T220I]) on cardiac arrhythmias, associated mortality, and ECG phenotypes in a multicohort observational study with >1 million individuals across 3 cohorts (FinnGen, UK Biobank, and Health 2000). The gene discussed is SCN5A; the disease is cardiac arrhythmia.