Intranasal topical steroids are commonly used for the treatment of olfactory dysfunction. Although the side effects are considered to be minimal, these have not been studied in detail. We examined the side effects of intranasal topical treatment with steroids in patients with olfactory dysfunction. We treated 62 patients with intranasal topical application of 0.1% betamethasone sodium phosphate (Rinderone) administered to the olfactory clefts. After treatment for 1 or 2 months, serum ACTH or cortisol was reduced in 42 (68% of total) patients, however, no clinical symptoms associated with steroid side effects were observed. The treatment was discontinued in 8 of these patients, who elected to withdraw from the treatment. In these patients, the serum ACTH or cortisol returned to normal a month after stopping the treatment. In the remaining 34 patients, the topical treatment was continued and in only 4 of these patients, minor steroid-associated side effects such as the sensation of facial swelling and facial hair thickening, appeared 2-5 months after beginning the treatment. These side effects disappeared within a month after stopping the treatment. Regarding the therapeutic efficacy, in 18 (78%) of the 23 patients who continued treatment for an average period of 5 months, steroid treatment significantly improved their olfactory dysfunction. In conclusion, although it is important to consider the potential side effects of long-term intranasal topical treatment with steroids for olfactory dysfunction, this treatment is a highly effective method against olfactory dysfunction with minimal side effects.