Thrombelastography (TEG) performed by an anesthesiologist provides a rapid assessment of coagulation at the bedside. TEG analyzing coagulation status of native whole blood is a more accurate test with a relatively good sensitivity and specificity than PT and aPTT. We experienced an unexpected coagulopathy during the perioperative period. The case was a 47-year-old male patient with blood type O who underwent elective spine surgery. Perioperative coagulation tests (PT, aPTT, BT, CT, etc.) were within normal limits. Anesthesia was induced with propofol 90 mg, vecuronium 8 mg and alfentanil 0.5 mg and maintained with 1.0 1.5 vol% enflurane and 50% N2O in O2. Then we performed acute normovolemic hemodilution (ANH) with monitoring pre- and post-hemodilutional TEG. Hemostasis was revealed as abnormal by a pre-hemodilution TEG (CI = -11.06) and post-hemodilution TEG (CI = -13.06). We managed this coagulopathy with blood components and drugs on the basis of a follow-up TEG so that abnormal hemostasis and TEG findings improved (CI = -4.35). We report a case where undetected coagulopathy was revealed and treated successfuly with TEG.