Seventy-seven paired serum samples from patients with known idiopathic sudden hearing loss (ISHL) were surveyed using viral serologic methods. Fifteen different viruses and Mycoplasma pneumonia were the agents tested. We determined an incidence of 65% (49/77) of documented significant seroconversions to one or more of the agents surveyed. Multiple agents were involved in 24 of the 49 positive cases we studied. Influenza virus Group B in 14 (18%) and rubeola in 12 (16%) were the most prevalent, followed by Herpes simplex type 1 in 6 (8%), mumps in 6 (8%), influenza Group A3 in 6 (8%), rubella in 5 (7%), and cytomegalovirus (CMV) in 5 (7%).