PURPOSE: To evaluate, using short-term follow-up MR imagings, the usefulness of gamma-knife radiosurgery in patients with acoustic schwannoma. MATERIALS AND METHODS: In 34 patients (M:F=11:23, aged 11 -69 years) with acoustic schwannoma, eleven of whom had undergone microsurgical resection prior to gamma-knife radiosurgery, we retrospectively reviewed the serial MR imaging findings obtained before and after this procedure. Analysis focused on post-surgical changes in tumor volume and intratumoral enhancement, and the follow-up period ranged from 3 to 44 months. RESULTS: Follow-up imaging revealed that after radiosurgery, tumor size had decreased in 17 cases (50%), was unchanged in 14 (41.2%), and had increased in three (8.8%). Local tumor control was achieved in 31 of 34 cases (91.2%). Objectively defined tumor shrinkage was seen within 3 to 24 (median, 12) months of treatment, the rate of shrinkage increasing with longer follow-up. Three to 16 (median, 6) months after treatment, loss of central tumor enhancement was evident in 28 cases(82.4%). In 25 of 28 patients with intratumoral necrosis (89.3%), tumors were either smaller of their size was unchanged. Three to six (mean, 3.6) months after treatment, five cases demonstrated a transient size increase. CONCLUSION: Gamma-knife radiosurgery effectively controlled the growth of acoustic schwannoma, and intra-tumoral necrosis appears to be a predictable sign for decreased tumor size.