Isolation of the subclavian artery is a very rare vascular anomaly in which the subclavian artery does not communicate with the aortic arch but instead is connected to the pulmonary artery by the ductus arteriosus. The subclavian steal phenomenon from the vertebrobasilar system into the pulmonary and subclavian arteries is usually persistent. We report an 18-month-old boy with an isolated left subclavian artery associated with tetralogy of Fallot and right aortic arch. Subclavian steal phenomenon in this patient resulted in symptoms of diminished pulse and blood pressure of the left arm. He underwent total correction of tetralogy of Fallot and ligation of the ductus arteriosus without reimplantation of the left subclavian artery. There was no substantial blood pressure difference between the upper limbs after the operation. In a patient with right aortic arch and diminished blood pressure or pulse in the left arm, isolation of the left subclavian artery should be included in the differential diagnosis.