Prolonged neuromuscular blockade is a relatively common complication of general anesthesia. Some previous reports have shown that positive serum anti-acetylcholine receptor antibody (AChR Ab) might contribute to this complication. We experienced a case of a 69-year-old woman with prolonged neuromuscular blockade after laparoscopic sigmoid colectomy under general anesthesia using rocuronium. A high level of anti-AChR Ab (45 nmol x l-1) was found in postoperative examination and the patient had progressive muscle weakness for six months after the operation. Although this patient had mediastinal tumor diagnosed as thymic carcinoma two years before the operation, preoperative clinical evaluation was negative for myasthenia gravis (MG) and the tumor was in remission at the operation. These observations suggest that preoperative measurement of anti-AChR Ab level might be recommended for patients with mediastinal tumor regardless of symptoms of MG and that neuromuscular blocking agents should be administrated carefully in anti-AChR Ab positive patients under monitoring of the neuromuscular blockade.