Vitamin K deficient hemorrhagic diathesis is well known as a cause of infantile intracranial hemorrhage. Its occurrence, however, as a post-surgical complication is rare and has never been reported previously. Two cases are presented here which illustrate the existence of such a hazard. Case 1. A 73-year-old woman admitted with subarachnoid hemorrhage (WFNS IV) underwent microsurgical exploration of a left internal carotid aneurysm, and neck clipping of the aneurysm was performed. She had an uneventful postoperative course, but her neurological condition deteriorated suddenly on the fifth postoperative day. CT scan revealed a large epidural hematoma. Case 2. A 6-year-old boy was admitted due to the dysfunction of a ventriculo-peritoneal shunt system that had previously been placed for hydrocephalus. This dysfunction was thought to be caused by meningitis. Twelve days after ventricular drainage and antibiotic therapy, sudden intraventricular hemorrhage occurred. In both cases, PT and APTT were markedly prolonged, FDP slightly increased and fibrinogen slightly decreased. SFMC was positive in case 2. After the administration of vitamin K, PT and APTT were immediately normalized. Recent reports emphasize the adverse effect of antibiotics that leads to vitamin K deficient hemorrhagic diathesis, especially, in patients in a cachectic state. In these two cases, such a cachectic condition was not observed. We presume that the cause of vitamin K deficiency would be, along with the administration of antibiotics, a preliminary condition of disseminated intravascular coagulation which is encountered in some neurological disorders including subarachnoid hemorrhage. We conclude that attention should be paid for these pitfalls in perioperative neurosurgical care.