Two patients who underwent gamma knife radiosurgery for ruptured cerebral arteriovenous malformations (AVM) developed cystic lesions at 78 and 111 months after undergoing treatment. Both patients presented initially with intracerebral hemorrhage. In one patient, the cystic lesion was discovered during routine follow-up imaging and clinical examination revealed homonymous hemianopsia; the second patient presented with seizure and the lesion was identified more than 9 years after radiosurgery. One patient underwent resection of the nidus and histologic analysis of the resected specimen showed vessels in various stages of obliteration. The present paper discusses the possible mechanism for the delayed development of cystic lesions, and the possibility that radiation-induced vascular changes may continue in a nidus even when angiography shows complete obliteration of the nidus.