Involvement of the retinal microvascular vessels is an almost inevitable consequence of long standing diabetes. In the only large scale epidemiology study of a Caucasian population Klein et al. found some degree of retinopathy in up to 97% of patients with insulin dependent diabetes mellitus (IDDM) after about 15 years diabetes duration and in about 60% of insulin treated patients of older onset, presumably non-insulin dependent diabetic patients (NIDDM). The presence of retinopathy does not indicate impending visual loss in all patients. Thus in IDDM the sight threatening forms of retinopathy is most commonly associated with neovascularization; up to 60% of patients develop this lesion after 20 years diabetes duration, while in the older age group new vessels are far less common. In clinic populations the commonest cause of visual loss in NIDDM is associated with macular oedema but in the population studies of Klein, prevalence of macular oedema was almost as common in IDDM as in NIDDM patients.