BACKGROUND Coronary artery fistulas are rare and typically congenital but can result from various acquired conditions. They involve abnormal connections between a coronary artery and a cardiac chamber or vessel, often creating a left-to-right shunt. Most coronary artery fistulas are asymptomatic and found incidentally, though larger fistulas can cause symptoms like ischemia or heart failure, requiring intervention. Management depends on the fistula size and clinical findings. Many small fistulas close spontaneously. CASE REPORT A 44-year-old woman with massive left atrial dilation and longstanding persistent atrial fibrillation due to an undiagnosed left-main-coronary-artery-to-left-atrium fistula after cardiothoracic surgeries (mechanical mitral valve replacement, childhood left main coronary aneurysm repair), presented with new-onset heart failure and reduced ejection fraction. Multimodal imaging detected a large (6.0 cm) left main coronary aneurysm with a connecting fistula to the left atrium resulting in a significant jet of turbulent flow on the atrial aspect of the mitral valve. The left atrium had become massive in size and the gradient across the prosthetic mitral valve was high at 10 mmHg. Hemodynamic parameters revealed pulmonary hypertension and a slightly low cardiac output. She underwent redo sternotomy, resection of the left main coronary artery aneurysm, and replacement with an 8 mm ringed Gore-Tex graft, closure of the left-main-coronary-artery-to-left-atrium fistula, and bovine pericardial patch exclusion of the left atrial appendage. Symptoms resolved after surgical repair. CONCLUSIONS Unexplained cardiac chamber enlargement and heart failure should prompt consideration of the presence of a coronary artery fistula, an important yet frequently overlooked treatable cause of heart failure.