We report a case of a 35-year-old woman who was diagnosed with a giant coronary artery aneurysm (CAA) with a fistula and followed-up using the contrast-enhanced computed tomography scan and 2-dimensional transthoracic echocardiography. The diameter and volume cubic content of the giant CAA had increased linearly during the 12-year follow-up period without any complications. As there are few reports about long-term follow-up of giant CAAs without a surgical intervention, the natural course of a giant CAA is still unclear. Thus, we may need to closely follow-up chronological changes of a giant CAA to decide the treatment strategy.