OBJECTIVE : The association of primary brain tumors with coexisting unruptured intracranial aneurysms (UIAs) has been debated, and treatment strategies have not yet been established. We evaluated the flow association between brain tumors and coexisting UIAs and the outcomes of patients with brain tumor who did not receive treatment of coexisting UIAs. METHODS : The medical records of patients with coexisting UIAs and brain tumors who had undergone surgical tumor resection were retrospectively analyzed to evaluate the incidence of UIAs according to tumor type, association of arterial feeder vessels and coexisting UIAs, and the occurrence of subarachnoid hemorrhage during the follow-up period for patients who had not receive treatment for the UIAs. RESULTS : Of the 55 patients, 21 (38.9%) had undergone treatment of UIAs and 33 (61.1%) had received regular follow-up examinations without UIA treatment. Two patients (6.1%) developed subarachnoid hemorrhage. Of the 33 patients with astrocytic or oligodendroglial tumors who had not undergone treatment of UIAs, 2 had experienced an interval increase. Flow-related UIAs were found to be significantly more common in patients with astrocytic or oligodendroglial tumors than in those with tumors of other histological types (P = 0.01). CONCLUSIONS : The incidence of UIAs among patients with primary brain tumors appears to be greater than that in the general population. An interval increase in aneurysm size and aneurysmal rupture was observed in 2 patients with astrocytic or oligodendroglial tumors. We would recommend perioperative treatment of flow-related UIAs in patients with astrocytic or oligodendroglial tumors and noninvasive evaluations such as magnetic resonance angiography if coexisting UIAs remain untreated.