We report a case of accidental intrathecal administration of Nucomyt(R)(acetylcysteine), a mucolytic agent used for intratracheal instillation. A 21-year-old healthy female with a complete syndactyly at the 4th and 5th toes was scheduled for a web release under spinal anesthesia. Immediately after accidental intrathecal administration of Nucomyt(R)2.4 ml instead of bupivacaine for spinal anesthesia, tonic extension developed at first in the lower extremities and then the upper extremities, too. Thiopental sodium 200 mg was injected intravenously twice. Endotracheal general anesthesia with N2O- enflurane-vecuronium was maintained for the operation for about two hours. For about 1 hour after emergence of general anesthesia, tonic extension intermittently developed in the lower extremities and was controlled with midazolam. Mild elevated blood pressure, tachycardia, tachypnea, high fever (up to 38.5degrees C) and respiratory acidosis occurred. Thirty minutes later, she became mentally clear with a normal neurologic examination. No delayed sequelae were detectable at a follow-up visit 1 month and 1 year after surgery.