A 3-yr-old girl was scheduled to undergo surgical repair of tetralogy of Fallot. She had no sign or data indicating an infectious disease, other than a slight dry cough for a few days prior to the proposed operation. During the induction of anesthesia with nitrous oxide, oxygen and sevoflurane, transient moist rale was noticed with a precordial stethoscope. Her trachea was intubated without any difficulty after the administration of pancuronium, followed by a chest auscultation, which revealed vesicular sound bilaterally but no rale. However, a chest X-ray taken after the right subclavian vein catheterization showed a massive hypoaeration in the upper left pulmonary region. The presence of the right-to-left intracardiac shunt made it impossible to detect the occurrence of atelectasis by a decrease in SpO2. Fiberoptic bronchoscopy showed no obstruction of the bronchus and no hypersecretion initially, but physical therapy and humidification made it possible to aspirate intratracheal sputum. Because there seemed to be an imbalance between the relatively uneventful induction of anesthesia and the relative resistance of atelectasis to authentic therapies, the operation was postponed, and the antibody to mycoplasma pneumoniae was titrated. The titer in the serum was 1:80, and increased to 1:560 6 days later. Chest X-rays revealed normal lung condition 3 days later, and she was given erythromycin, 800 mg.day-1 for 2 weeks. We conclude that we should be alert to possible asymptomatic mycoplasma infection, which potentially makes patients susceptible to atelectasis during the perioperative period.