Mitral valve prolapse (MVP) is a common cardiac abnormality, and there are numerous documented cases of intraoperative dysrhythmias. However, the authors describe what is believed to be the first case of atrial fibrillation in a patient with an undiagnosed MVP who was given a subarachnoid block. A possible clinical sequencing for the subarachnoid block is offered. The differential diagnosis, etiology, pathology, and pathophysiology of MVP are discussed. Also detailed are the management of intraoperative dysrhythmias. Physicians should be aware, as in the case reported, that a new-onset atrial fibrillation can occur, even without previously diagnosed MVP.