Because of its anatomical location, mediastinal tumor is frequently accompanied by airway stenosis and signs indicating compression of the heart and large vessels. For this reason, a patient with this tumor often requires an urgent operation. We recently conducted such an operation on a girl 2 year and 7 month old in whom a giant tumor originating from the posterior mediastinum had caused dyspnea. In this case, anesthesia was induced with ketamine. Intubation was carried out while the girl was semiawake. No muscle relaxants were used, and spontaneous ventilation was partially preserved. During the operation, anesthesia was maintained with oxygen, nitrous-oxide and halothane, without using muscle relaxants. Although intratracheal bleeding, caused by manipulation of the tumor, aggravated the blood gas data, this could be coped with by elevating the oxygen concentration in the inspired gas. During the operation, respiratory control with 10cmH2O PEEP was carried out to cope with atelectasis from lung compression by the tumor. The postoperative course was excellent. From anesthetic management of this case we emphasize the following points: (1) preoperative assessment of the relationship between posture and dyspnea and assessment of the locational relationships of the tumor, heart, vessels and trachea, using CT, ultrasonography, bronchoscopy, etc; (2) utilizing a pulse oximeter, monitoring CO2 in expired gas and monitoring CVP during operation; (3) avoidance of the use of muscle relaxants before the improvement of the symptoms arising from tumor-caused compression; and (4) close respiratory care after operation.