Abstract Aims Postoperative atrial fibrillation (POAF) is the most common complication of cardiac surgery and has been associated with increased postoperative morbidity and hospital costs. The Posterior left pericardiotomy for the prevention of AtriaL fibrillation After Cardiac Surgery (PALACS) trial found that posterior pericardiotomy significantly reduced the incidence of POAF (17% vs. 32%, P < 0.001). We present the protocol for The Effect of Posterior Pericardiotomy on the Incidence of Atrial Fibrillation After Cardiac Surgery–Extended Follow-Up study (PALACS-EF): a prospective, extended follow-up of the original PALACS trial. The aim of PALACS-EF is to gain more data regarding the effect of posterior pericardiotomy on postdischarge clinical outcomes. The primary outcome is the time to the first occurrence of the composite of all-cause mortality or hospital cardiovascular readmission. The key secondary outcome is the time to the first occurrence of the composite of all-cause mortality and all-cause hospital readmission. Hospital readmission, myocardial infarction, stroke, transient ischaemic attack, heart failure, systemic embolism, or new arrhythmias with onset since 30-day follow-up will also be recorded. Methods and results All 420 patients enrolled in the PALACS trial will be included; extended follow-up will be conducted via telephone by blinded research personnel utilizing a standardized script to ensure uniformity and completeness of follow-up. If an event has occurred, documentation will be obtained, and an independent adjudication committee blinded to group assignment will adjudicate outcome events. Results will be reported when a median follow-up of 5 years is achieved. Conclusion PALACS-EF will provide data to answer the question of whether posterior pericardiotomy improves postdischarge outcomes in patients undergoing cardiac surgery, and it will provide information on the relationship between POAF and adverse postdischarge outcomes including mortality, hospitalization, heart failure, and stroke. Registration PALACS: NCT02875405, PALACS-EF: NCT05903222