Nearly 90% of severe visual loss among patients with senile macular degeneration can be ascribed to the neovascular form of the disease. The Senile Macular Degeneration Study demonstrated that argon laser photocoagulation can reduce the risk of severe visual loss by more than 60% among patients with choroidal new vessel membranes 200 microns or greater from the center of the foveal avascular zone (FAZ). The Krypton Photocoagulation Study, also supported by the National Eye Institute, is evaluating whether krypton red laser photocoagulation can reduce the risk of visual loss in SMD patients with new vessels within 200 microns of the center of the FAZ.