We performed immunopathologic studies on pars plana specimens obtained by biopsy in patients with diabetes mellitus type I or II and by autopsy in diabetic patients and normal subjects. Frozen sections were treated with several antisera, including anti-IgG, complement components, and major histocompatibility complex antigens, as well as anti-factor VIII to detect vascular structures. The results showed IgG in a linear pattern at the basal pole of pigment epithelial cells and complement deposits of C3c, C3d, and C4 at the same location and in the stroma. HLA-DR expression was found at the level of the pigmented cells. These data suggest that some autoimmune processes may be involved in proliferative diabetic retinopathy at the level of the pigment epithelium, but it is unknown whether they are an epiphenomenon of neovascularization or if they play a role in its initiation.