Eight cases of cerebral cyst formation among 50 patients (= 16%) with malignant supratentorial gliomas treated by surgery, megavoltage radiation, and multiple-agent chemotherapy are reported. Five of them developed signs of intracranial hypertension or progressive neurological deficit, while in three patients cerebral cysts were detected by CT without clinical deterioration. At operation or autopsy, or both, the large fluid-filled, smooth-walled cysts were lined by glio-mesenchymal scar tissue with no or little tumour recurrence in five, while three patients showed large recurrent tumour masses associated with necrosis and cyst formation. Clinical signs or CT evidence, or both, of cerebral cysts developed 4 to 12 months (average 10 months) after the first craniotomy, and 3 to 9 months after termination of radiotherapy, usually after the second to fourth course of polychemotherapy. The cystic cavities which are attributed to increased necrosis and other effects of radiation and cytostatic treatment, may mimic tumour progression or recurrence, and cerebral abscess, but are usually recognized by CT. Surgical treatment produced transient clinical improvement in 5 patients, but usually did not prevent the fatal outcome of the disease, which in these paticrosis and other effects of radiation and cytostatic treatment, may mimic tumour progression or recurrence, and cerebral abscess, but are usually recognized by CT. Surgical treatment produced transient clinical improvement in 5 patients, but usually did not prevent the fatal outcome of the disease, which in these paticrosis and other effects of radiation and cytostatic treatment, may mimic tumour progression or recurrence, and cerebral abscess, but are usually recognized by CT. Surgical treatment produced transient clinical improvement in 5 patients, but usually did not prevent the fatal outcome of the disease, which in these patients occurred 3 weeks to 6 months after surgical treatment of cyst formation, their life span ranging from 9 to 22 months. The pathogenesis and clinical problems related to cerebral cysts arising following multimodality treatment of malignant brain tumours are discussed.