Purpose To examine whether slowed rod-mediated dark adaptation (DA) in adults with normal macular health at baseline is associated with the incidence of age-related macular degeneration (AMD) 3 years later. Design Prospective cohort. Participants Adults aged ≥60 years were recruited from primary care ophthalmology clinics. Both eyes were required to be step 1 (normal) on the Age-Related Eye Disease Study 9-step AMD classification system based on color fundus photographs graded by experienced and masked evaluators. Methods Rod-mediated DA was assessed at baseline in 1 eye after a photobleach using a computerized dark adaptometer with targets centered at 5° on the inferior vertical meridian. Speed of DA was characterized by the rod-intercept value, with abnormal DA defined as rod-intercept ≥12.3 minutes. Demographic characteristics, best-corrected visual acuity, and smoking status were also assessed. Log-binomial regression was used to calculate unadjusted and adjusted risk ratios (RRs) and associated 95% confidence intervals (CIs) for the association between baseline DA and incident AMD. Main Outcome Measures Presence of AMD at the 3-year follow-up visit for the eye tested for DA at baseline. Results Both baseline and follow-up visits were completed by 325 persons (mean age, 67.8 years). At baseline, 263 participants had normal DA with mean rod-intercept of 9.1 (standard deviation [SD], 1.5), and 62 participants had abnormal DA with mean rod-intercept of 15.1 (SD, 4.0). After adjustment for age and smoking, those with abnormal DA in the tested eye at baseline were approximately 2 times more likely to have AMD in that eye (RR, 1.92; 95% CI, 1.03-3.62) by the time of the follow-up visit, compared with those who had normal DA at baseline. Conclusions Delayed rod-mediated DA in older adults with normal macular health is associated with incident early AMD 3 years later, and thus is a functional biomarker for early disease. The biological relevance of this test is high, because it assesses translocation of Vitamin A derivatives across the retinal pigment epithelium and Bruch's membrane, 2 tissues with prominent age- and AMD-related pathology.