OBJECTIVE: To determine the prevalence rate of otosyphilis and Lyme-associated labyrinthine disease among patients presenting with idiopathic, progressive sensorineural hearing loss and, thus, to provide recommendations regarding the diagnostic workup. STUDY DESIGN: A prospective cohort study of patients presenting to a university hospital otology clinic with idiopathic, progressive sensorineural hearing loss between 1998 and 2006. SUBJECTS AND METHODS: Cases in which hearing loss was clearly attributable to other causes were excluded, leaving 181 patients (mean age 55.6 years). History, physical examination, autoimmune laboratory tests, syphilis and Lyme serology, audiometric assessment, and MRI were performed in all cases. RESULTS: Nine cases (4.9%) of otosyphilis and none of Lyme disease were diagnosed by serological tests. CONCLUSION: Routine serological testing for otosyphilis, but not Lyme disease, is recommended for all cases of idiopathic, progressive sensorineural hearing loss.