The efficacy and safety of low-molecular-weight heparins (LMWH) as postoperative venous thromboembolism prophylaxis has been demonstrated in more than 60 clinical trials, including more than 20,000 patients (1). However, recent reports of spinal hematoma occurring spontaneously and in association with regional anesthesia (2,3) have generated concern regarding the safety of spinal or epidural anesthesia in patients receiving LMWH. This review will discuss the chronology of events leading to the current situation, as well as examine the possible factors contributing to the increased risk of spinal hematoma in patients receiving LMWH. heparin, significant anti-Xa activity is still present 12 hours after injection. The clearance of LMWH is primarily renal. The plasma half-life of LMWH is approximately two to four times longer than that of standard heparin and increases in patients with renal failure. The anticoagulant effects of standard heparin are neutralized by an equimolar dose of protamine. Because of the reduced protamine binding to LMWH fractions, only the anti-IIa activity of LMWH is completely reversed, whereas anti-Xa activity is not fully neutralized.Both anti-IIa and anti-Xa activity may return up to 3 hours after protamine reversal, possibly because of tile release of additional LMWH from the subcutaneous depot (22).