We examined the effectiveness of sodium fluoride in treatment of cochlear otospongiosis in ninety-four patients with cochlear otosclerosis and ninety-eight patients with stapedial otosclerosis and sensorineural hearing loss. The drug halted or slowed the progression of sensorineural hearing impairment in 63% of the patients with cochlear otosclerosis and 46% of the patients with stapedial otosclerosis. The single factor that best predicted which patients would respond most favorably to treatment was rate of progression before treatment. Sodium fluoride therapy was successful for 79% of the patients losing their hearing at a rate of 5 dB or more per year at one or more of the speech frequencies. The finding that patients with more rapid rates of progression responded most favorably to sodium fluoride therapy suggests that patients with the most active otospongiotic processes will be the most responsive to treatment.