A 23-year-old asymptomatic woman was referred to our hospital for further examination of a systolic ejection murmur with fixed splitting of the second heart sound auscultated at the third left sternal border. Initial echocardiography could not detect the cause. Subsequently performed low-dose computed tomography, however, ruled out the possibility of any congenital heart diseases, but revealed a markedly shortened anteroposterior diameter of the chest, which led us to a diagnosis of straight back syndrome. A vertically oriented "pancake" appearance of the heart, straight vertebral column, and compression of the right ventricular outflow tract were clearly demonstrated on the reconstructed images.