Radiosurgery (RS) is a new technique that is being developed worldwide to treat a variety of central nervous system (CNS) diseases. We report preliminary results concerning the first 35 patients treated at our institution between October 1989 and December 1990. These patients presented with pituitary adenomas (PA: 12 patients), primary CNS tumors with different histologies (11 patients), arteriovenous malformations (AVM: 6 patients), acoustic neurinomas (3 patients) and CNS metastases (3 patients). All patients were treated with a linear accelerator and received a single dose of 20 Gy delivered on the 70% isodose curve localized stereotactically by mean of either a CT scan or angiography. Preliminary results with a median follow-up of 26 months were as follows. AVM had totally disappeared in 3 patients after 12 and 17 months, and remained stable in 3 at 6 and 26 months. In 3 PA patients, the adenoma was partially regressed on the CT scan and, in 2 patients, the visual field was moderately improved; none experienced clinical improvement due to hormonal hypersecretion. One patient with neurinoma experienced clinical improvement, but the CT scan remained unchanged in the 3 patients. Of the patients with primary or secondary tumors, 8 exhibited a reduced lesion diameter on the CT scan, but 3 died later of progressive disease. Complications were not rare. In 7 patients, delayed reactions were observed: one case of cerebral edema reversed under corticotherapy; and 6 cases of neurological impairment due to brain necrosis, reversed in only one. RS appears to be an interesting approach for the treatment of deeply located AVM and for small acoustic neurinomas.(ABSTRACT TRUNCATED AT 250 WORDS)