Objective More than 50% of atrial fibrillation surgery occurs in the setting of mitral valve surgery. Despite this, no risk models have been validated for concomitant arrhythmia surgery. The purpose of the present study was to quantify the additional risk of performing the Cox-maze IV procedure for patients undergoing mitral valve surgery. Methods From January 2002 to June 2011, 213 patients with mitral valve disease and preoperative atrial fibrillation underwent mitral valve surgery only (n = 109) or in conjunction with a Cox-maze IV procedure (n = 104). The operative mortality for the mitral valve procedure alone was predicted for each group using the Society of Thoracic Surgeons perioperative risk calculator. The risk attributed to the added Cox-maze IV procedure was calculated by comparing the predicted mortality rate of an isolated mitral valve procedure and the actual mortality rate of mitral valve surgery with a concomitant Cox-maze IV procedure. Results For patients not undergoing a Cox-maze IV procedure, the predicted and actual postoperative mortality rate was 5.5% and 4.6% (5 of 109), respectively. For patients receiving mitral valve surgery and a concomitant Cox-maze IV, the predicted and actual postoperative mortality of the mitral valve procedure was 2.5% and 2.9% (3 of 104), respectively, and not significantly different. Patients not offered a Cox-maze IV procedure had significantly more serious comorbidities. Conclusions For patients with atrial fibrillation and mitral valve disease undergoing mitral valve surgery, the decision to offer a concomitant Cox-maze IV procedure will be influenced by the underlying comorbid conditions. Nonetheless, in selected lower risk patients, the addition of a Cox-maze IV procedure did not significantly affect the procedural mortality.