THIS STUDY EXAMINES the surgical practice at the Wessex Neurological Centre over the 5-year period from 1989 to 1993 to determine the incidence of postoperative hematoma and to identify risk factors for a perioperative bleeding disorder. The study includes only those postoperative hematomas (at any site) that followed and were related to a neurosurgical operation and were surgically evacuated. The study is prospective for the year 1993 and retrospective for the preceding years. Over the 5 years, 6668 operations were performed and 71 postoperative hematomas were surgically evacuated, accounting for an overall rate of 1.1% of operations. The records were available for 69 cases. The most frequent diagnosis leading to postoperative hematoma was meningioma surgery with a rate of 6.2% of cases (13 of 211); followed by craniotomy for trauma, 3.7% (7 of 192); aneurysm surgery, 2.6% (11 of 428); and intrinsic supratentorial tumors, 2.2% (10 of 451). Postoperative hematomas were intraparenchymal in 43% of cases, subdural in 5%, extradural in 33%, mixed in 8%, and confined to the superficial wound in 11%. The overall mortality was 32% (37% for intraparenchymal and 12% for extradural). Risk factors for a perioperative bleeding disorder were present in two-thirds of the patients. Administration of antiplatelet agents (aspirin and nonsteroidal anti-inflammatory drugs) was the most commonly associated risk factor. At least 75% of these identified risk factors could potentially have been avoided or corrected.