Generalized retinal arteriolar narrowing is an important sign of systemic hypertension, and a lower arteriolar:venular diameter ratio predicts the risk of hypertension. We investigated whether this association was based on arteriolar or venular diameters or both. This study was based on the prospective population-based Rotterdam Study (1990-1993) and included 1900 participants (> or =55 years of age) of whom 739 persons had normal blood pressure (systolic <120 mm Hg and diastolic <80 mm Hg) and 1161 prehypertension (systolic 120 to 139 mm Hg or diastolic 80 to 89 mm Hg). For each participant, retinal arteriolar and venular diameters were measured on digitized images of 1 eye. After a mean follow-up of 6.6 years, 808 persons developed hypertension, defined as either systolic blood pressure > or =140 mm Hg or diastolic blood pressure > or =90 mm Hg or use of antihypertensive medication. Adjusted for age, gender, follow-up time, body mass index, smoking, diabetes mellitus, total and high-density lipoprotein cholesterol, C-reactive protein, and intima-media thickness, arteriolar narrowing was associated with an increased risk of hypertension (odds ratio per SD: 1.38; 95% CI, 1.23 to 1.55); for venular narrowing this was less striking (OR: 1.17; 95% CI, 1.04 to 1.32). Each SD decrease in the arteriolar:venular diameter ratio significantly increased the risk of hypertension by 24%. To examine the effect of baseline blood pressure, we stratified persons into those with "normal blood pressure" or "prehypertension." Within these strata, arteriolar narrowing was still related to incident hypertension. These data show that both retinal arteriolar and venular narrowing may precede the development of systemic hypertension.