We have observed retroperitoneal hematoma after trigger point injections of quadratus lumborum in a patient with chronic low back pain. Severe flank pain and dyspnea was observed three hours after injection of local anesthetic and steroid to the trigger point of quadratus lumborum muscle. There was fuge hematoma in abdominal CT image around the right kidney, which displaced and compressed the kidney anteriorly. Following infusion of contrast media, extravasation through renal vein and IVC was notified. Patient had a past history of having been treated with platelet aggregation inhibitor and lower dose aspirin treatment after cerebral ischemia for a year, but coagulative function was within normal range. Patient was admitted 12 days for bed rest, pain control and transfusion. We need to take greater care with a frequent aspiration and exact direction of needle, during trigger point injection of quadratus lumborum, particu right side, to avoid vascular injury.