A prospective audiometric evaluation of 69 hospitalized febrile patients in Sierra Leone, West Africa, revealed a sensorineural hearing deficit (SNHD) in 14 (29%) of 49 confirmed cases of Lassa fever and in 0 of 20 febrile controls. An SNHD was present in nine (17.6%) of 51 people who had evidence of previous Lassa virus infection. Twenty-six of 32 local residents who had previously sustained a sudden deafness had antibody titers to Lassa virus of 16 or greater, compared with six of 32 matched controls. Lassa fever is associated with an incidence of SNHD, which considerably exceeds that previously reported with any other postnatally acquired infection, and accounts for a prevalence of virus-related hearing impairment in the eastern province of Sierra Leone that is greater than that reported from anywhere else in the world.