We obtained follow-up data on 84 eyes with age-related macular degeneration and poorly defined angiographic leakage presumed to represent choroidal neovascularization. A poorly defined neovascular membrane was presumed to be present when subsensory retinal fluid was present in association with choroidal leakage on fluorescein angiography and in which the extent of leakage was not well defined. Among the 84 eyes, the average initial visual acuity was 20/80. In 75 (89%) of 84 eyes, the leakage involved the foveal center at initial presentation. At follow-up (average, 28 months; range, six to 53 months), the average visual acuity was 20/250; the final acuity declined at least three but less than six lines in 18 eyes (21%) and six or more lines in 35 eyes (42%). There was a statistically significant difference in the percentage of eyes that developed moderate or severe visual loss among eyes that progressed to disciform scarring compared with eyes that continued to manifest poorly defined leakage without evidence of scarring. Given that most severe visual loss associated with macular degeneration can be attributed to consequences of neovascular membranes and that many membranes associated with age-related macular degeneration are poorly defined, our study results support the possibility that poorly defined neovascular membranes represent a major cause of severe visual loss among the elderly in the United States.