Although many series of patients with cerebral arteriovenous malformations (AVMs) treated radiosurgically have been published, there has been little information on cysts appearing several years after irradiation. Herein, we discuss the incidence, mechanisms and predictability of late cyst formation based on our personal experiences, as well as reported patients. The incidence of this complication, though generally considered to be 0.5% or less, may be higher than assumed. Although a breakdown of the blood-brain barrier is likely to play a major role in the formation process, the hematoma cavity itself may have the potential to become a cyst. A radiation-induced lesion appearing several years after irradiation and persisting for several years thereafter may be a warning sign of late cyst formation. Long-term follow-up, particularly using neuroimaging techniques, is necessary even after the 'treatment goal' has been achieved.