Purpose: To investigate the interleukin (IL)-6 levels, IL-8 levels, and cellular composition of the vitreous humor in patients with proliferative diabetic retinopathy (PDR), proliferative vitreoretinopathy (PVR), and traumatic PVR. Methods: Vitreous samples from 14 patients with PDR, 10 patients with PVR, and 10 patients with traumatic PVR were analyzed. Fifteen cadaver eyes were used as controls. Cytokine levels were measured by ELISA. Results: Elevated IL-6 levels were detected in the vitreous of 12 (85.7%) of the PDR patients, eight (80%) of the PVR patients, and all (100%) of the traumatic PVR patients. None of the control IL-6 results were elevated. Vitreous IL-8 levels were elevated in 12 (85.7%) of the PDR patients, six (60%) of the PVR patients, all (100%) of the traumatic PVR patients, and one (6.7%) of the control eyes. Cytological examination of the vitreous specimens revealed a predominance of macrophages (50%) in the PDR samples and a predominance of retinal pigment epithelial (RPE) cells (60%) in the PVR samples. In contrast, neutrophils predominated (88%) in the traumatic PVR samples. Conclusion: The findings suggest that IL-6 and IL-8 may be involved in the pathogenesis of PDR, PVR, and traumatic PVR. High proportions of RPE cells and macrophages are associated with elevated IL-6 and IL-8 levels in the vitreous of PDR and PVR patients; however, the fact that these cells are not predominant in traumatic PVR suggests that different immune response mechanisms may be active in the pathogenesis of these disorders.