The authors report a case of partially thrombosed giant aneurysm which was secondarily infected with purulent meningitis. The relationship between the infection of the aneurysm, the rapid growth of the aneurysm and the development of severe cerebral edema was discussed. A 53 year-old man was admitted on September 1, 1986, with a diagnosis of bacterial meningitis. On his admission, his body temperature was 39 degrees C, and he showed mental confusion but no neurological deficits. Laboratory data revealed signs of infection in white blood cell count, CRP, and erythrocyte sedimentation rate. Computerized tomographic (CT) scan and magnetic resonance (MR) imaging showed a massive round mass with perifocal edema measuring 40mm in the maximum diameter in the left paramedian frontal region. T1 weighted MR image also showed the presence of pus accumulation in the left ventricle. Cerebral angiography demonstrated a giant aneurysm at the distal portion of the azygos anterior cerebral artery, and irregular narrowing of both the supraclinoid segment of the carotid artery and its main branches indicating arteritis due to purulent meningitis. The patient was treated with ventricular drainage and administration of antibiotics. Culture of the purulent CSF was negative. The patient's lab data, CSF finding and neurological status improved progressively. However, follow-up CT scan and angiogram a month later showed enlargement of the aneurysm, dilatation of the patent lumen and perifocal edema. On October 8, the patient suddenly became comatose with anisocoria. A CT scan showed massive edema with marked midline shift. Emergency bifrontal craniotomy was carried out, and clipping was completed after removal of the thrombosed portion of the aneurysm, and thromboendarterectomy of the aneurysmal neck.(ABSTRACT TRUNCATED AT 250 WORDS)