Severe hip pain developed in a patient who was receiving anticoagulation therapy of heparin sodium and warfarin sodium (Coumadin) for femoral vein phlebitis. The patient had exquisite tenderness but no ecchymoses along the sciatic nerve. A drop in the hemoglobin level showed a large volume loss of blood. Retroperitoneal hematoma with lumbar plexopathy was suspected, but a computerized tomographic scan revealed instead hemorrhage into the gluteal muscles. The anticoagulation therapy was corrected with the addition of protamine sulfate and vitamin K; the patient's pain lessened and the hemoglobin level stabilized. Computerized tomography allowed clear visualization of the hematomas in the buttock, and invasive studies were avoided. Hip pain and a falling hemoglobin level in a patient receiving anticoagulation therapy should alert one to the possibility of bleeding into the retroperitoneal space, thigh, or buttock.