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To investigate the prognostic value of peripheral retinal nonperfusion in patients with diabetic retinopathy using ultra-widefield fluorescein angiography (UWFA). A cross-sectional study included 78 treatment-naive eyes with nonproliferative and proliferative diabetic retinopathy (NPDR and PDR). Eyes were divided into three groups: mild/moderate NPDR (n = 31), severe NPDR (n = 31), and PDR (n = 16). Three nonperfusion variables were calculated reflecting the proportion of nonperfused to visible retina based on initial UWFA: central nonperfusion (CNP) index, peripheral nonperfusion (PNP) index, and PNP ratio. The relationships between these indices and central subfield thickness (CST) and spectacle-corrected visual acuity (SCVA) were evaluated. CNP and PNP indices were significantly higher in the PDR group vs. mild/moderate NPDR group (p = 0.007 and 0.008, respectively) but not in the PDR group vs. severe NPDR group (p = 0.149 and p = 0.535, respectively). A significant linear correlation was found between the CNP and PNP indices in both severe NPDR and PDR groups (R2 = 0.141, p = 0.041, and R2 = 0.311, p = 0.025, respectively). Nonperfusion predominance was not statistically correlated with the presence of macular edema (p = 0.058) or disorganization of retinal inner layers (p = 1). In the severe NPDR group, there was a moderately positive correlation between CNP index and CST (rs = 0.496, p = 0.019) and no correlation between CNP index and SCVA when controlling for CST (p = 0.160). In the PDR group, a strong negative correlation between PNP ratio and CST was found (rs = −0.659, p = 0.014), but no correlation was observed between CNP index, CST, and SCVA. In the PDR group, a positive correlation was found between PNP index, PNP ratio, and SCVA (rs = 0.549, p = 0.027, and rs = 0.626, p = 0.010, respectively), even after controlling for CST (rs = 0.599, p = 0.040). Higher amounts of retinal nonperfusion are seen in patients with more severe retinopathy. Increased CNP is associated with macular thickening and subsequent vision loss. Having predominantly PNP was independently associated with worse VA, regardless of macular thickness. Further studies are needed to investigate the role of PNP in vision loss in diabetic retinopathy.