Among participants with any cardiovascular disease who had an ICD and were at high risk for ventricular arrhythmias, a treatment-induced increase in plasma potassium levels led to a significantly lower risk of appropriate ICD therapy, unplanned hospitalization for arrhythmia or heart failure, or death from any cause than standard care. (Funded by the Independent Research Fund Denmark and others; POTCAST ClinicalTrials.gov number, NCT03833089.).