There is not an established consensus on the treatment of coronary steal syndrome caused by unligated thoracic side branches of internal mammary artery. Occlusion of thoracic side branch is not recommended without objective evidence of ischemia. This case report presents transbrachial coil occlusion of thoracic side branch of left internal mammary artery causing silent ischemia in a diabetic patient with atypical chest pain and palpitation.