The Senile Macular Degeneration Study (SMDS) is a multicenter controlled clinical trial designed to answer the question, "Is argon laser photocoagulation useful in preventing severe visual loss in eyes with evidence of senile macular degeneration and a choroidal neovascular membrane outside the fovea?" Eligible patients were assigned randomly to a "treatment" group or a "no treatment" group. Recruitment in the SMDS has been terminated because, after 18 months of follow-up, 60% of untreated eyes v 25% of treated eyes had experienced severe visual loss. Follow-up of all patients continues to assess long-term results of treatment. Patients with senile macular degeneration who are at risk for the development of choroidal neovascularization should be examined periodically so that if symptomatic neovascularization develops, treatment can be considered before irreversible visual loss occurs.