A 63-year-old woman with a 2-month history of abdominal distension received diagnostic laparotomy under general anesthesia. The chest X-ray one week preoperatively demonstrated slight left pleural effusion, but she did not show any dyspnea on preanesthetic interview. General anesthesia was induced with propofol, ketamine and fentanyl. Sp(O2) decreased after suction of ascites, but it improved with PEEP Spontaneous respiration developped after the operation, but tidal volume was not enough and respiratory pattern was irregular. Train of four was 100%, and bronchofiberscopy had no suspicions findings. The chest X-ray and TEE revealed extended left hemilateral pleural effusion. Seven hundred seventy ml of hemorrhagic pleural fluid was suctioned. Respiratory pattern improved and the endotracheal tube was removed smoothly. We consider that we diagnosed her state only from her subjective symptoms without considering objective symptoms. Besides we had to explain a possibility of a unanticipated serious respiratory distress. In such a case, more accurate and objective diagnostic procedures are required.