The purpose of the study is to compare the effect of intravitreal injections of ranibizumab and aflibercept on systemic VEGF protein levels in DME patients in a detailed time course.

Inclusion Criteria:

• Type 1 or Type 2 diabetes mellitus
• Visual impairment predominantly due to DME.

Exclusion Criteria:

• Stroke or myocardial infarction less than 3 months prior to screening.
• Presence of uncontrolled systolic blood pressure or diastolic blood pressure
• Renal failure requiring dialysis or renal transplant or renal insufficiency
• Untreated diabetes mellitus
• Use of any systemic anti-VEGF drugs
• Use of systemic or inhaled corticosteroids for at least 30 consecutive days within 3 months prior to screening.
• Women who are pregnant or breast feeding or who are menstruating and capable of becoming pregnant* and not practicing a medically approved method of contraception

For either eye:

• Any active periocular or ocular infection or inflammation
• Uncontrolled glaucoma
• Neovascularization of the iris or neovascular glaucoma
• History of treatment with any anti-angiogenic drugs

For study eye:

• Atrophy or fibrosis involving the center of the fovea at the time of screening or baseline.
• Cataract (if causing significant visual impairment), planned cataract surgery during the study period, vitrectomy, aphakia, glaucoma surgery, severe vitreous hemorrhage, rhegmatogenous retinal detachment, proliferative retinopathy or choroidal neovascularization or macular edema of any other cause than DME
• Irreversible structural damage within 0.5 disc diameter of the center of the macula
• Panretinal laser photocoagulation within 6 months prior to randomization.
• Focal/grid laser photocoagulation within 3 months prior to randomization.
• Any intraocular procedure (including cataract surgery, Yttrium-Aluminum-Garnet capsulotomy) within 3 months prior to baseline or anticipated within the next 6 months following
• Topical ocular corticosteroids administered for at least 30 consecutive days within 3 months prior to screening.
• Application of intravitreal corticosteroids in vitreous within 6 months prior to screening. Prior application of fluocinolonacetonid releasing implant in vitreous within 36 months prior to screening.

For fellow eye

- Retinal or choroidal neovascularization or macula edema of any cause

Other protocol-defined inclusion/exclusion criteria may apply.