ABSTRACTONE HUNDRED EIGHTY patients affected by cerebral arteriovenous malformations (AVMs) underwent radiosurgical treatment in our department. One hundred fifty-three patients have been treated with complete irradiation of the entire AVM nidus. In 27 patients (with large and/or three-dimensional irregular target volumes), only part of the nidus was covered with a dose adequate for obliteration. Follow-up ranged from 88 to 1 months (mean, 43.1 mo). Angiographic control was performed at 12, 24, and 36 months until complete obliteration was attained. The complete obliteration rate was 46% at 1 year and 80% at 2 years. We observed 15 hemorrhages after treatment, and five patients died from them. No bleeding took place after complete angiographic obliteration. The aim of this study is to evaluate the effect of irradiation on bleeding risk after radiosurgery and before complete obliteration. Inclusive parameters of patients considered at risk were as follows: 1) all patients in the time lapse between irradiation and demonstrated complete angiographic obliteration; 2) all patients in the time lapse between irradiation and definitive treatment either by surgery or embolization; and 3) all patients in the time lapse between irradiation and death. These groups include all irradiated patients who still had incompletely obliterated AVMs. They were stratified starting from 0 time (the date of radiosurgery), and the hemorrhages were evaluated every 6 months. In totally irradiated cases, the bleeding risk decreased from 4.8% in the first 6 months after radiosurgery to 0% starting from the 12th month of the follow-up. In partially irradiated cases, the bleeding risk increased from 4% in the first 6 months to 12 to 10% from the 6th to the 18th month and decreased to 5.5% from the 18th to the 24th month; no bleeding was observed after the 24th month.