Eleven year-old boy was scheduled for right encephalo-duro arterial synangiosis (EDAS). He received left EDAS 4 months ago, and there was no problem during the perioperative period. Mild cardiomegaly was suspected in his preoperative chest PA. After intubaton, we couldn't hear the breathing sound of left chest, and the saturation went down to 95%. There was large opaque hemithorax on emergency chest AP, and on the fiberoptic bronchoscopic examination, left main stem bronchus (LMSB) was observed slit-like appearance. Echocardiography revealed severely dilated right atrium and ventricle. In his chest CT, the LMSB was nearly obliterated. The patient was diagnosed as primary pulmonary hypertension, and received full sedation, controlled ventilation, and NO gas. However, the blood pressure of this patient was not maintained in spite of using high-dose inotropics. He expired two days later and this case warns that suspected mild cardiomegaly might hide more ominous sign of primary pulmonary hypertension.