Between January 1974 and December 1986, 5,975 patients underwent open heart surgery at National Institute of Cardiology "Ignacio Chávez". We reviewed all open heart cases carried out at this Center. Eleven patients (1.8/1000) presented unilateral sudden hearing loss following cardiopulmonary bypass surgery. Pure tone audiograms, speech reception threshold, discrimination testing, tympanometry and stapedius reflex testing were carried out. We then studied the relationship between the auditory deficit and the type of surgical procedure, length of time on the pump, preexisting ear disease, coexistence of diabetes, use of ototoxic drugs and occurrence of hemodynamic complications. There are 3 possible causes for hearing loss in cardiopulmonary bypass: 1) Microembolism from the cardiopulmonary bypass pump to the arteries that supply only the cochlea. 2) Decreasing cerebral flow, or 3) Microhemorrhage of the internal ear. Ototoxic drugs are used frequently, but the hallmark of these drugs is bilateral toxicity, and all the cases under discussion have been unilateral. Unilateral sudden deafness after open heart surgery constitutes another complication, fortunately rare and benign in most cases. Further studies should be undertaken to clarify the pathophysiology of this entity.