Postoperative chronic subdural hematoma (CSH) following craniotomy developed in only four of 372 patients undergoing craniotomy for aneurysm surgery (1 patient) and brain tumor surgery (3 patients) between April, 1991 and November, 1993, an incidence of only 1.1%. There were three males and one female, aged from 32 to 66 years (mean 56 yrs). The period between craniotomy and development of CSH ranged from 3 to 5 months (mean 4.3 mos). Postoperative hematomas were located on the operative sides in three patients and the contralateral side in the other. Early postoperative computed tomographic scans found subdural fluid collection in all patients. Magnetic resonance images showed linear meningeal enhancement in all patients. Postoperative CSH may be caused by mixture of blood in subdural cerebrospinal fluid collection which persists due to reduced brain elasticity and wide subarachnoid membrane opening resulting in neomembrane formation and finally development of hematoma.