Objective: It is unclear if estrogen replacement therapy (ERT)-has an effect on the retinal circulation. In the current study, we examine the association of ERT, female reproductive factors, and retinal vascular caliber. Design: Populati on-based crosssectional study. Participants: Women participants aged 43 to 84 y ears living in Beaver Dam, Wisconsin. Methods: Retinal photographs of participan ts taken at the baseline examination were digitized, and the diameters of arteri oles and venules were measured using awell-established technique. Estrogen repl acement therapy and female reproductive factors were ascertained by interview. M ain Outcome Measures: Retinal arteriolar and venular diameters. Results: Of the 2469 women participants with data for analysis, 10.5%were current users of ERT and 7.4%were past users. After adjusting for age, blood pressure (BP), body mas s index, smoking, and other factors, women who were current users of ERT had nar rower retinal arteriolar and venular diameters than those who were past users or never used, with mean arteriolar diameters of 167.6 μm for current users, 170. 8 μm for past users, and 170.9 μm for those who never used (P=0.009) and mean venular diameters of 239.9 μm for current users, 244.0 μm for past users, and 243.9 μm for those who never used (P=0.02). There was a signifi-cant trend of increasing narrowing for both arterioles (P trend, 0.01) and venules (P trend, 0 .007) with increasing duration of ERT. Associations were somewhat stronger in yo unger women and women without a history of hypertension and cigarette smoking. F emale reproductive factors (e.g., age of menarche and pregnancy) were not associ ated with retinal vessel diameters. Conclusions: Estrogen replacement therapy is associated with narrower retinal vessel diameters, independent of BP and other vascular factors. In the current study, we examine the association of ERT, female reproductive factors, and retinal vascular caliber. Design: Populati on-based crosssectional study Participants: Women participants aged 43 to 84 y ears living in Beaver Dam, Wisconsin. Methods: Retinal photographs of participan ts taken at the baseline examination were digitized, and the diameters of arteri oles and venules were measured using awell-established technique. Estrogen repl acement therapy and female reproductive factors were ascertained by interview. M ain Outcome Measures: Retinal arteriolar and venular diameters. Results: Of the 2469 women participants with data for analysis, 10.5% were current users of ERT and 7.4% were past users. After adjusting for age, blood pressure (BP), body mas s index, smoking, and other factors, women who were current users of ERT had nar rower retinal arteriolar and venular di ameters than those who were past users or never used, with mean arteriolar diameters of 167.6 μm for current users, 170. 8 μm for past users, and 170.9 μm for those who never used (P = 0.009) and mean venular diameters of 239.9 μm for current users, 244.0 μm for past users, and 243.9 μm for those who never used (P = 0.02). There was a signifi-cant trend of increasing narrowing for both arterioles (P trend, 0.01) and venules .007) with increasing duration of ERT. Associations were somewhat stronger in yo unger women and women without a history of hypertension and cigarette smoking. F emale reproductive factors (eg, age of menarche and pregnancy) were not associ ated with retinal vessel diameters. Conclusions: Estrogen replacement therapy is associated with narrower retinal vessel diameters, independent of BP and other vascular factors.