A 21-year-old woman underwent correction of congenital thoracic kyphoscoliosis. The procedure was completed in two stages. The first stage involved right thoracotomy and anterior vertebrectomy. Following positional obstruction of her right-sided chest drain she developed a very large, left-sided pneumothorax in the immediate postoperative period. No attempt at central venous line placement had been made on the left side. Breach of the midline at vertebrectomy may allow pleural gas to cross and cause a contralateral pneumothorax.