A 16-year-old male presented with a hemopneumothorax following a gunshot wound to the left chest. He was treated effectively with closed suction drainage for 48 hours when he suddenly complained of severe left chest pain. Preoperative diagnosis was suggestive of a traumatic diaphragmatic hernia but operative findings were confirmatory of a rare hernia through the foramen of Bochdalek. Chest X-ray showing a gas-filled viscus above the diaphragm is diagnostic. Increased abdominal pressure generated when he was first hit by the bullet and aggravated by increased negative intrapleural pressure resulting from suction drainage of the hemothorax is the probable mechanism of the herniation.