The COST line corresponds to the junction between the photoreceptor tips and the apical processes of the retinal pigment epithelium, where photoreceptor OS disc membranes are continuously shed for renewal.6 Thus, the appearance of the COST line may reflect the normal function of the photoreceptor OSs more closely than the IS/OS junction. In fact, in all of the AZOOR cases we have recently examined, the COST line was always absent in the region of IS/OS abnormalities, suggesting that the abnormality of the COST line may precede that of the IS/OS junction. In our 2 cases, the fundus appeared normal and the IS/OS junction was clearly observed in the region of the COST line abnormality for 50 and 18 months after the onset. The focal macular ERGs, however, were markedly reduced in the affected areas. In the OCT images, the cone photoreceptor dysfunction corresponding to the region of scotoma could be detected only by the abnormality of the COST line.