[en] Purpose: The rate of hearing loss in a population before and after irradiation was investigated to determine the effect of irradiation on hearing impairment. Methods and materials: In 72 patients with vestibular schwannoma who received fractionated stereotactic irradiation from 1992 to 1999, 21 had had their hearing levels examined 3 months or more before the treatment. The mean time between the initial examination and treatment was 18.6 months (range: 3-89 months), and the mean time between treatment and the last follow-up was 24.2 months (12-69 months). Thirty-six to 50 Gy in 20-25 fractions over 5 to 6 weeks was given using an X-ray beam from a linear accelerator. Pure tone average (PTA) was measured using the mean hearing level at five frequencies, and the annual rate of hearing loss was defined as [(hearing loss in PTA(dB))/(follow-up period (months)x12)]. Results: The actual cumulative curve of decrease in tumor size of 2 mm or more was 38.3% at 2 years and 80.0% at 3 years. The mean of hearing loss in PTA was 11.6±10.3 dB (-1 to 35 dB) from the initial examination to the start of irradiation and 11.9±14.4 dB (-14 to 37 dB) from the start of irradiation to the last follow-up. The mean annual rates of hearing loss before irradiation and in the 1st, 2nd, 3rd and 4th years after irradiation were, respectively, 18.6, 11.2, 6.2, 5.1, and 5.0 dB/year. The annual rates of hearing loss in the 2nd year (P=0.025) and 3rd year (P=0.018) were significantly slower than the rate before irradiation. Conclusions: The mean annual rate of hearing loss was higher before irradiation than after irradiation, and hearing loss slowed rather than accelerated after irradiation. Although hearing loss after the treatment was usually permanent, fractionated stereotactic irradiation was suggested to be effective to lower the rate of hearing loss