The objectives of this proof-of-concept study are to investigate whether doxycycline can 1) slow the deterioration or improve retinal function and/or 2) induce regression, or slow progression, of diabetic retinopathy. The tests will be performed in the Ophthalmology Departments of the Penn State College of Medicine and Glostrup Hospital, Copenhagen, Denmark. The 24 month proof-of-concept clinical study will involve a prospective, randomized, double-masked clinical trial including 60 adult patients with type 1 or type 2 diabetes who have severe non-proliferative diabetic retinopathy (ETDRS level 53E) or mild or moderate proliferative diabetic retinopathy (retinal and /or optic disk neovascularization less than the "high-risk" ETDRS level 61 or 65), neovascularization of the disc or neovascularization elsewhere >1/2 disc area and in whom panretinal photocoagulation is not imminently required in the ophthalmologist's judgment.