 Impaired retinal blood flow has been implicated in the pathogenesis of diabetic retinopathy. Patel et al. 1992 showed that retinal blood flow increases with the level of diabetic retinopathy. Grunwald et al. 1996 reported that patients with insulin dependent diabetes mellitus IDDM of relatively short duration have increased retinal blood flow, even before the onset of diabetic retinopathy. On the other hand the data of Bursell et al. 1996 indicate that IDDM patients have reduced retinal blood flow, when they have normal blood glucose levels, but this study may have considerable methodological limitations. Acute elevations of blood glucose levels, however, result in an increase in retinal blood flow Grunwald et al. 1987, Bursell et al. 1996. Based on previous experimental data the investigators hypothesize that ocular blood flow is increased in early diabetes and reduced at later stages of the disease. Previous studies have demonstrated that metabolic conditions such as hyperglycemia influence outcome parameters and thereby might have confounded results regarding ocular blood flow in diabetic retinopathy. The investigators will therefore study patients with IDDM during euglycemic conditions. 