To describe the cross-sectional relationship between retinal arteriolar and venular diameters with age and blood pressure. A population-based study was conducted in Beaver Dam, Wisconsin (n=4926, age range, 43-84 years). Retinal photographs of right eyes taken at the baseline examination (1988-90) were digitized. All arterioles and venules located in the area between one-half and one disc diameter from the optic disc margin were measured with a computer-based program. These measurements were combined to provide the average diameters of retinal arterioles and venules of each eye, and the association with age and blood pressure (BP) was analyzed. After controlling for gender, hypertension, diabetes, serum glucose and lipids, cigarette smoking, and body mass index, retinal arteriolar diameters were found to be decreased by 2.1 microm (95% confidence interval [CI], 1.5-2.7) for each decade increase in age, and by 4.4 microm (95% CI, 3.8-5.0) for each 10-mm Hg increase in mean arterial BP. The association of narrowed retinal arterioles and higher BP was stronger in younger persons. For each 10-mm Hg increase in mean arterial BP, arteriolar diameters decreased by 7.0 microm in persons aged 43 to 54 years but by only 2.5 microm in persons aged 75 to 84 years. In contrast, retinal venular diameters narrowed with increasing age but not with increasing BP. Retinal arteriolar diameters are narrower in older persons and in persons with higher BP, independent of other factors. The weaker association of retinal arteriolar diameters and BP in older people may reflect greater sclerosis of the retinal arterioles, preventing a degree of narrowing with higher BP similar to that seen in younger persons.