out DR and a control group of non-diabetic normal subjects. We evaluated the subjects seen in our department from August 2004 to July 2006. Approval for the use of human subjects was obtained from the Institutional Review Board of the Samsung Medical Center. All participants provided informed consent prior to their participation in the study. All of the subjects underwent complete ophthalmologic examinations, which included a best-corrected visual acuity measurement with the Snellen visual acuity chart, slit-lamp biomicroscopy, dilated fundus examinations using a 90-diopter pan-fundus lens, and fluorescein angiography. On the basis of fundoscopy and angiography, the eyes of the diabetic subjects were classified as no DR, mild/moderate/severe non-proliferative diabetic retinopathy (NPDR), and PDR [13]. Diabetics with DR should exhibit DR of any stage in at least one eye. If both eyes were the same stage, the right eye was selected. If eyes demonstrated different stages of DR, the eye with the more advanced stage was included. The exclusion criteria included preretinal/vitreous hem- orrhage, advanced cataracts, significant corneal opacities, and traction affecting the macular or peripapillary zone. Patients having undergone intravitreal injections, laser pho- tocoagulation (focal or panretinal), or intraocular surgery other than cataract extraction conducted more than 6 months before were also excluded, as these interventions may cause retinal thickness changes. Finally, the patients had no other retinal diseases, including macular degener- ation, retinal vein occlusion, or hypertensive retinopathy. A single ophthalmic photographer blinded to the sub- jects’ conditions conducted OCT (StratusTM, Carl Zeiss, Dublin, CA, USA) scanning in all eyes following pupillarg dilation. The macula was initially scanned along the hori- zontal and vertical meridians using a standard, linear cross-hair pattern, with a scan length of 6 mm centered through the fovea. In cases in which macular edema pre- cluded adequate foveal localization of the scan, the OCT was centered on the patient’s fixation. For the peripapillary zone, three linear scans and one circular scan were conducted while a nasally- positioned fixation target was presented to the subject. The three peripapillary linear scans included one horizontal scan (nasal to optic disc) and two vertical scans taken superior and inferior to the optic disc, with a scan length of 6 mm starting from within one third of the disc diameter from the disc margin in each, in an effort to avoid including peripapillary atrophy within the scanned zone. The peripapillary circular scan was obtained concen- tric to the optic disc, resulting in a peripapillary ring of 6 mm in diameter along the margin of the optic disc. The OCT parameters of this study, or the calculated mean thicknesses of the obtained scans, were abbreviated as horizontal (H) and vertical (V) in the macula, and superi- or (S), inferior (I), nasal (N), and circular along the margin of the optic disc (D) in the peripapillary zone, as shown in Fig. 1. HY Cho, et al. Diabetic Retinopathy and PeripapillaryThickness Fig. 1. Optic coherence tomography scans were performed at the macula and peripapillary zones, indicated by the line superimposed on the fundus photograph. Horizontal (H) and vertical (V) scans of the macula were centered through the fovea. Superior (S), inferior (I) and nasal (N) scans of the peripapillary zone originated within the one-third of disc diameter from the disc margin. All linear scans were 6 mm in length. The peripapillary circular scan (D) was ob- tained concentric to the optic disc, with a diameter of 6 mm. In addition, central macular thickness (CMT) at the deepest portion of the foveal pit was determined using manually- positioned OCT software-controlled cursors. For all cross- sectional OCT scans, retinal thickness was defined as the distance between the internal limiting membrane and the retinal pigment epithelium, as was determined automatically by the OCT analysis software. The mean retinal thickness in each location for the six scans was calculated using image processing software (Image/J, National Institutes of Health, Bethesda, MD, USA). Each individual scan was reviewed by a blinded observer (DHL) for aberrant placement of the inner and outer retinal borders. If a portion of the thick- ness measurements were determined to be incorrect, man- ual correction was applied using image processing soft- ware to adjust the inner and outer bands. At every follow-up visit after the initial examination, bi- omicroscopic fundus examinations were conducted by one examiner (SWK), who was blinded to the patients` eligibility. Additionally, when indicated, panretinal photocoagulation (PRP) was also conducted during the follow-up period. PRP was considered in accordance with the recommendations of the Diabetic Retinopathy Study [14]. In patients presenting with clinically significant macular edema, macular focal laser photocoagulation was administered in accordance with the guidelines established in the Early Treatment of Diabetic Retinopathy Study [15, 16]. Eyes with DR were, thus, classified into two groups, designated the PRP and No-PRP groups. The PRP group included eyes which had required PRP during the 6-month follow-up period after the baseline examination, and the No-PRP group included eyes which had not required PRP. 17