OBJECTIVE: Cerebral arteriovenous malformations (AVMs) are rare cerebral vascular lesions that are associated with high morbidity and mortality from hemorrhage; however, stereotactic radiosurgery (SRS) is a well-validated treatment modality. Few reports have delineated a subgroup of patients who develop delayed chronic encapsulated expanding hematomas (CEEHs) despite angiographic evidence of AVM obliteration following radiosurgery. In this report, the authors performed a retrospective review of more than 1000 radiosurgically treated intracranial AVM cases to delineate the incidence and management of this rare entity. METHODS: Between 1988 and 2019, 1010 patients with intracranial AVM underwent Gamma Knife SRS at the University of Pittsburgh Medical Center. In addition to a review of a prospective institutional database, the authors performed a retrospective chart review of the departmental AVM database to specifically identify patients with CEEH. Pertinent clinical and radiological characteristics as well as patient outcomes were recorded and analyzed. RESULTS: Nine hundred fifty patients with intracranial AVM (94%) had sufficient clinical follow-up for analysis. Of these, 6 patients with CEEH underwent delayed resection (incidence rate of 0.0045 event per person-year). These patients included 4 males and 2 females with a mean age of 45.3 ± 13.8 years at the time of initial SRS. Four patients had smaller AVM volumes (4.9-10 cm3), and 3 of them were treated with a single SRS procedure. Two patients had larger-volume AVMs (55 and 56 cm3), and both underwent multimodal management that included staged SRS and embolization. Time to initial recognition of the CEEH after initial SRS ranged between 66 and 243 months. The time between CEEH recognition and resection ranged from 2 to 9 months. Resection was required because of progressive neurological symptoms that correlated with imaging evidence of gradual hematoma expansion. All 6 patients had angiographically confirmed obliteration of their AVM. Pathology revealed a mixed chronicity hematoma with areas of fibrosed blood vessels and rare areas of neovascularization with immature blood vessels but no evidence of a persistent AVM. All 6 patients reported persistent clinical improvement after hematoma resection. CONCLUSIONS: CEEH after SRS for AVM is a rare complication with an incidence rate of 0.0045 event per person-year over the authors' 30-year experience. When clinical symptoms progress and imaging reveals progressive enlargement over time, complete resection of a CEEH results in significant clinical recovery. Knowledge of this rare entity facilitates timely detection and eventual surgical intervention to achieve optimal outcomes.