Diabetic retinopathy often has no early warning signs. Even macular edema, which may cause vision loss more rapidly, may not have any warning signs for some time. In general, however, a person with macular edema is likely to have blurred vision, making it hard to do things like read or drive. In some cases, the vision will get better or worse during the day.

In the first stage which is called non-proliferative diabetic retinopathy (NPDR) there are no symptoms, it is not visible to the naked eye and patients will have 20/20 vision. The only way to detect NPDR is by fundus photography, in which microaneurysms (microscopic blood-filled bulges in the artery walls) can be seen. If there is reduced vision, fluorescein angiography can be done to see the back of the eye. Narrowing or blocked retinal blood vessels can be seen clearly and this is called retinal ischemia (lack of blood flow).

Macular edema may occur in which blood vessels leak contents into the macular region can happen at all stages of NPDR. The macular edema symptoms are blurring, darkening or distorted images with not the same between two eyes. 10 percent of diabetic patients will get vision loss related with macular edema. Optical Coherence Tomography can show areas of retinal thickening (fluid accumulation) of macular edema.

On the second stage, as abnormal new blood vessels (neovascularisation) form at the back of the eye as a part of proliferative diabetic retinopathy (PDR), they can burst and bleed (vitreous hemorrhage) and blur vision, because the new blood vessels are weak. The first time this happens, it may not be very severe. In most cases, it will leave just a few specks of blood, or spots, floating in a person's visual field, though the spots often go away after a few hours.

These spots are often followed within a few days or weeks by a much greater leakage of blood, which blurs vision. In extreme cases, a person will only be able to tell light from dark in that eye. It may take the blood anywhere from a few days to months or even years to clear from the inside of the eye, and in some cases the blood will not clear. These types of large hemorrhages tend to happen more than once, often during sleep.