A 33-year-old woman with an anomalous left coronary artery arising from the pulmonary artery who had undergone Takeuchi repair at age 7 years presented for evaluation. The Takeuchi procedure creates an aortopulmonary window and an intrapulmonary tunnel that baffles the left coronary artery to the aorta. A mediastinal mass was identified as a giant aneurysm of the left coronary artery resulting in compression of the pulmonary artery and left upper pulmonary vein. The patient underwent open repair with patch closure at the aortic entrance of the left coronary Takeuchi repair and resection and evacuation of the aneurysm. A saphenous vein graft to the left anterior descending artery was performed. Postoperative echocardiography demonstrated normal left ventricular function. This is the first reported case of giant aneurysm formation after Takeuchi repair. The reported complications have included the development of pulmonary artery stenosis at the intrapulmonary baffle, baffle leak, decreased left ventricular function, and mitral regurgitation. In conclusion, late complications of the Takeuchi procedure are common, underscoring the importance of lifelong follow-up at a center with experience in treating coronary anomalies.