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A 54-year-old man with a history of coronary artery bypass grafting (CABG) presented with chest pain and was found to have non-ST-segment elevation myocardial infarction. Left heart catheterization with coronary angiography demonstrated 100% occlusion of the right internal mammary artery (IMA) to the right coronary artery graft in its midsegment and a patent left IMA to the left anterior descending graft. An unusually large extensive fistulous collateral formation was observed between the right IMA and the left IMA to the pulmonary arterial system, causing left to right shunting. His angina was attributed to substantial coronary steal caused by the shunt. The patient refused any further intervention or surgery and opted for medical treatment. As a complication of CABG, IMA to pulmonary artery (PA) fistulas are rare. Thus far, more than 20 cases have been reported; most have been unilateral. This is the second reported case to date of bilateral IMA-PA fistula formation after CABG. An IMA-PA fistula should be considered in the differential diagnosis of patients presenting with chest pain after CABG and can be diagnosed by selective angiography of IMA grafts.