NEUROSURGICAL PATIENTS ARE at high risk for the development of thrombosis and thromboembolism. We compared the perioperative clotting factor and coagulation parameters of 20 patients undergoing elective craniotomy for brain tumors to those of 20 patients undergoing elective abdominal surgery. We also measured the levels of plasma arginine vasopressin to determine if changes in this hormone might be associated with changes in clotting factors, activated partial thromboplastin times, or bleeding times. The results demonstrated a significant reduction in partial thromboplastin times and bleeding times in the neurosurgery group, which began at the initiation of surgery and lasted to the end of the study (12 h postoperatively). Elevations in factor assays and plasma arginine vasopressin occurred in both groups during surgery, but there were no differences between the neurosurgical and abdominal surgical patients, except with Factor IX levels, which were elevated only in the neurosurgical patients. Serum osmolality and hemoglobin levels were significantly higher in the neurosurgical cohort. These results suggest that there are hemostatic differences between neurosurgical patients with brain tumors and abdominal surgery patients that cannot be explained solely by elevations in plasma arginine vasopressin or the clotting factors measured; these differences may be the consequence of perioperative variables such as dehydration and hyperosmolality.