The relationship between viral seroconversions and idiopathic sudden hearing loss (ISHL) is studied. Compared with our control group, the incidence of viral seroconversions is greater among ISHL patients, both for single and multiple viral infections. There was a significantly greater number of patients with seroconversions to mumps, rubeola, varicella-zoster, cytomegalovirus, and influenza B. We were unable to find a relationship between viral seroconversion and type or degree of hearing loss, vertigo, or chance for recovery. The incidences of viral conversion and sudden hearing loss track one another closely, suggesting that viral infection is a major cause of ISHL. During this 3-year study in Boston, ISHL was most prevalent in the spring.