This retrospective study of 146 ears with long-term follow-up after otosclerosis surgery evaluated the stability of hearing results, the incidence of sensorineural hearing loss, and the effect of fluoride treatment. Follow-up was at least 15 years (mean, 25.2 yr; range, 15-44 yr). There were 97 large fenestra stapedectomy operations, 23 lateral canal fenestrations, 7 mobilizations, and 19 revision stapes operations. The level of air-bone gap achieved at surgery remained stable over time; the mean deterioration rate was only 0.2 dB per year. Profound sensorineural hearing loss ( > or = 65 dB bone conduction average) at the most recent follow-up occurred in 13 ears (8.9%). Such hearing loss occurred in all operative groups. Mean bone conduction average immediately postoperatively was significantly higher in these ears than in others in the study. This finding indicates that a mixed hearing loss at surgery is a factor that increases the risk of later profound cochlear loss. Only 3 percent of ears with pure conductive hearing loss, but 28 percent of patients with mixed hearing loss at surgery eventually suffered profound cochlear loss. Sodium fluoride was used to treat 11 ears with progressive cochlear loss. The rate of bone conduction hearing deterioration decreased in all ears after treatment, and none developed profound hearing loss. Follow-up after the first postoperative year is not necessary if pure conductive hearing loss is present at surgery. Annual follow-up with audiograms is recommended if a mixed hearing loss is present. Fluoride treatment is recommended if inner ear hearing loss progresses.