Reported herein is a case of ruptured anterior cerebral aneurysm followed by primary intracranial malignant lymphoma. A 73-year-old female was admitted to our hospital on October 20, 1986, with a severe subarachnoid hemorrhage. Microsurgery was performed immediately on the day of admission because cerebral angiography revealed bilateral distal anterior cerebral artery aneurysms. Consciousness gradually deteriorated due to cerebral vasospasm from the 3rd day following surgery. To protect the brain, corticosteroid (methylprednisolone 1500 mg, dexamethasone 252 mg) was administered for 26 days after surgery. In spite of treatment, in March 1987 neurological examination revealed akinetic mutism, but CT scan did not reveal any evidence of tumor. CT scan performed on May 2, 1987, revealed a mass lesion in the left frontotemporal lobes and left basal ganglia. Histological diagnosis of malignant lymphoma (diffuse, large-cell type) was obtained by stereotactic biopsy. Radiotherapy (41 Gy of 60Co) was carried out. The CT scan performed on August 26, 1987 revealed that the abnormal enhanced lesion had disappeared. However, neurological findings had not remarkably improved. The patient died of renal failure in February 1988, but tumor recurrence had not been noted in CT scans. Autopsy could not be performed. Whole body CT scan and 67Gd scintigram did not reveal any other tumors except in the brain. Ruptured cerebral aneurysm followed by primary intracranial malignant lymphoma is very rarely reported. In such cases, which have unusual clinical courses and CT findings, we recommend stereotactic biopsy to treat this kind of radiosensitive tumor. The connection between malignant lymphoma and immunosuppression has been considered.(ABSTRACT TRUNCATED AT 250 WORDS)