Purpose: To study the relation between posterior vitreous detachment (PVD) and progression of diabetic retinopathy (DR), based on our observation that proliferative DR is rare in patients with complete PVD. Methods: The medical records of 403 patients with diabetes were reviewed for the relation between progressive DR and the status of PVD and HbA_1c over 3 years. PVD was classified into none, complete PVD with collapse, complete PVD without collapse, partial PVD with a thickened posterior vitreous cortex, and partial PVD without a thickened posterior vitreous cortex. DR was classified into none, simple, preproliferative, or proliferative. When it became more extensive or when laser treatment or vitreous surgery was performed, the DR was considered progressive. Results: Progression of DR over 3 years occurred in 128/292 (43.8%) eyes with no PVD, 0/14 (0%) eyes with complete PVD with collapse, 2/8 (25%) eyes with complete PVD without collapse, 15/15 (100%) eyes with partial PVD with a thickened posterior vitreous cortex, and 19/74 (25.7%) eyes with partial PVD without a thickened posterior vitreous cortex. Progression of DR occurred significantly more frequently in eyes with partial PVD with a thickened posterior vitreous cortex compared to eyes with complete PVD with collapse (p<0.0001). HbA_1c, did not differ significantly between these two groups (6.9 ± 0.9% and 7.5 ± 0.9%, respectively; p = 0.14), although HbA_1c was significantly higher (p = 0.04) in patients with progressive DR (78 ± 1.8%) than in patients without progressive DR (7.5 ± 1.5%). Conclusion: Complete PVD is a strong negative risk factor for DR. The PVD status in patients with diabetes should be evaluated.