The effect of sodium fluoride treatment in patients with otospongiosis has been evaluated in a prospective clinical double-blind, placebo-controlled study of 95 patients. The results showed a statistically significant greater deterioration of hearing loss in the placebo group than in the group actively treated with 40 mg of sodium fluoride daily. These results support the view that sodium fluoride can change otospongiotic, active lesions to more dense, inactive otosclerotic lesions.