The effects of relatively low dose gamma knife irradiation on acoustic schwannoma were evaluated. The signal intensity change and tumor shrinkage on magnetic resonance (MR) images, change in hearing, and complications in 28 patients (mean age 47.0 +/- 13.6 yrs) were studied. Three patients had bilateral tumors. Six were already deaf when treated. The maximum tumor diameter was 35 mm. The mean dose delivered to the tumor was 12.1 +/- 1.6 Gy at the periphery, and 25.2 +/- 4.3 Gy at the center. The mean follow-up time was 16 months and the longest 24 months. Lowering of the MR signal intensity in the tumor center appeared after 3 months at earliest but generally after 6 months. Signs of tumor shrinkage appeared within 12 months on average. Cyst in the tumor enlarged rapidly after treatment in two patients. The percentage of hearing preservation was 85% (17/20) at 3 months, 80% (16/20) at 6 months, 72% (13/18) at 9 months, 75% (12/16) at 12 months, 67% (8/12) at 15 months, 60% (6/10) at 18 months, and 50% (2/4) at 24 months. Subtle changes in hearing were detected by speech tone audiometry. Temporary facial numbness and weakness was seen in one patient each. No patient had lower cranial nerve paresis. Relatively low dose gamma knife radiosurgery is effective in suppressing growth of acoustic schwannoma with preservation of hearing.