The study included 562 patients with headache who visited our clinic from January 1988 to December 1993. In these patients, the possibility of subarachnoid hemorrhage was denied from CT findings and color of cerebrospinal fluid by lumbar puncture. Cerebral aneurysm was found in 52 out of 562 patients (9.3%), and minor leak was found during 46 surgery cases in 8 patients with cerebral aneurysm. These 8 patients (32-51 years old, 5 males and 3 females, period from onset to hospital visit; 1-6 days) were examined concerning characteristics of headache and concomitant symptom, cerebrospinal fluid and operative findings. Headache attack occurred at rest in 6 patients (75%) and suddenly in all patients. It continued over 24 hours in 7 patients (87.5%). All patients felt that they had never experienced such a headache. Pulsating headache was observed in 5 patients (62.5%). Nausea and vomiting were complications in all patients. The site of headache was in the forehead region, and circumorbital pain was a complication in four patients. Six patients (75%) had unilateral pain, and 5 patients (62.5%) had headache at the same side as the cerebral aneurysm. Increase of cell count ( > or = 10/mm3) in cerebrospinal fluid was found in 4 patients (50%). In all cases, lymphocytes were dominant. Arachnoid adhesion and hypertrophy with hemorrhage were found during surgery in 4 patients (50%). In these patients, the period from onset to surgery (average 16.5 days) was longer than that of the patients with hemorrhage alone (average 6 days). The site of the hemorrhage was located in the cistern of the cerebral aneurysm in all cases. In patients suffering sudden headache, screening for cerebral aneurysm should be thoroughly performed though early findings on CT and cerebrospinal fluid indicated no abnormalities.