A case of bronchogenic cyst associated with a partial pericardial defect is reported. Bronchogenic cysts are not so rare in incidence, but they are more rare when associated with a pericardial defect, the first case being reported by Rusby and Sellors in 1945. Recently, we experienced such a rare case of a bronchogenic cyst with a partial pericardial defect. The patient is a 39-year-old female and she was found to have a left anterior mediastinal mass during routine chest X-ray. During the operation, we detected partial pericardial defect after removal of the mediastinal mass. The pericardial defect was repaired with a Gore-Tex Membrane. The pathological examination of the mass showed a bronchogenic cyst. The patient had an uneventful hospital course.