Prolonged choroidal filling on fluorescein angiography in age-related macular degeneration is thought to indicate diffuse thickening of Bruch's membrane. To test the importance of this clinical sign, we reviewed the evolution of disease in eyes of patients with good visual acuity and a readable transit phase of fluorescein angiography at the time of recruitment into a longitudinal study of age-related macular degeneration. Ninety-six eyes satisfied these criteria. Of the 32 eyes with prolonged choroidal filling, 12 (38%) lost two or more lines, of visual acuity by two years, whereas only nine of 64 (14%) eyes with normal choroidal filling did so. The difference was caused by the higher incidence of geographic atrophy in the first group. The proportion of eyes that developed subretinal neovascularization was the same in the two groups, and no pigment epithelial detachments occurred. These findings indicate that this clinical sign has implications concerning visual prognosis in age-related macular degeneration.