We reviewed the initial experience with 331 consecutive patients treated during the first 2 years of operation of the University of Pittsburgh Gamma Unit. Among 178 patients with vascular malformations, 161 had angiographically demonstrable arteriovenous malformations, and 17 had angiographically occult malformations. Patients with tumors included 123 with histologically benign, predominantly extra-axial neoplasms and 30 with malignant, predominantly intra-axial neoplasms. When the arteriovenous malformation nidus could be covered with at least the 50% isodose line and a dose to the margin greater than or equal to 20 Gy, 60% were obliterated within 1 year. Patients with benign tumors and well-circumscribed malignant tumors did not experience tumor growth within the radiosurgical treatment volume. The role of radiosurgery is expanding to include primary treatment of larger lesions than was formerly possible and adjunctive treatment of more complex arteriovenous malformation and tumors.