The Diabetic Retinopathy Study (DRS) and the Early Treatment Diabetic Retinopathy Study (ETDRS) showed that laser was effective in treating:-

i) diabetic macular oedema
Macular photocoagulation is effective for treating clinically significant macular oedema (defined as thickening of the retina and/or hard exudates within 500 microns of the fovea, or retinal thickening within 1.5 mm of the fovea if the area of the thickening is of the same order as the area of the optic disc).  The laser does not usually improve vision, merely reducing the amount of visual loss associated with macular oedema.

ii) proliferative diabetic retinopathy especially patients with high risk characteristics.  These include:-
i) new vessels at the optic disc (NVD) that cover greater than 25% of it’s surface area
ii) any NVD with vitreous or preretinal haemorrhage,
iii) and new vessels in other locations of 50% disc area size associated with vitreous or preretinal haemorrhage.
Laser in less severe cases is effective in reducing the risk of these high risk characterisitics (which need urgent attention)  and therefore long-term visual loss.  Early treatment is particularly useful for patients with type 2 diabetes (reducing the risk of severe visual loss and the need for vitrectomy by 50%), whereas the timing of treatment is not so important in patients with type 1 diabetes.