Objective: Retinal vessel diameters, in particular larger venular diameters, have been associated with cerebrovascular disease. Larger retinal venular diameters may reflect cerebral ischemia. The authors investigated whether arteriolar oxygen saturation (SaO(2)) and total cerebral blood flow (CBF), indicators of cerebral oxygen supply, are associated with retinal arteriolar or venular diameters. Design: Cross-sectional study performed within the population-based Rotterdam Study. Participants: Randomly selected participants aged 55 years or older (n = 696), who underwent both an eye examination and brain magnetic resonance imaging (MRI). Methods: Arteriolar oxygen saturation was determined by pulse oximetry on the right index finger. Cerebral blood flow was assessed using a phase-contrast MRI sequence that measured the flow in the basilar and both internal carotid arteries. Brain volume was measured to express CBF per 100 ml brain volume. Retinal arteriolar and venular diameters were measured on digitized fundus color transparencies on 1 eye of each participant. Regression models were used to investigate the association of SaO(2) and CBF with retinal vessel diameters. Main Outcome Measures: Mean retinal arteriolar and venular diameters (in micrometers). Results: Lower SaO(2) was associated with larger venular diameters. Persons with SaO(2) less than 96% (n 113) had on average 5 mu m larger venular diameters compared with those with SaO(2) of 96% or more (n = 583; age- and gender-adjusted mean difference, 5.6 mu m; 95% confidence interval, 1.2-10.0). Cerebral blood flow was not related to venular diameters when analyzed separately. Additional analyses showed that the association between SaO(2) and venular widening was confined to participants within the lowest tertile of CBF. No associations were found between SaO(2) or CBF and arteriolar diameters. Additional adjustment for established cardiovascular risk factors did not change the results. Conclusions: An association of lower SaO(2) with larger retinal venular diameters was observed, in particular in the presence of lower CBF. These findings suggest that venular widening may reflect a lower oxygen supply, especially to the brain.