The Macular Photocoagulation Study randomized controlled clinical trials designed to determine whether argon laser photocoagulation is effective for preventing or delaying severe visual loss in eyes with an extrafoveal choroidal neovascular membrane secondary to senile macular degeneration (Senile Macular Degeneration Study [SMDS]), ocular histoplasmosis (Ocular Histoplasmosis Study [OHS]), or idiopathic neovascularization (Idiopathic Neovascularization Study [INVS]). Eligible patients were assigned randomly to a "treatment" group or to a "no treatment" group until recruitment was terminated in 1982 (SMDS) and 1983 (OHS and INVS) because of demonstrated short-term treatment benefit. As of Aug 31, 1985, three or more years of scheduled follow-up examinations had been completed for 208 (88%) of 236 eyes in the SMDS, 203 (77%) of 262 eyes in the OHS, and 51 (76%) of 67 eyes in the INVS. The relative risk of experiencing severe loss of vision after three years in eyes initially assigned to the no treatment group in comparison with eyes assigned to the argon laser photocoagulation group was 1.4 in the SMDS (95% confidence interval [CI]: 1.1 to 1.9), 5.5 in the OHS (95% CI: 3.9 to 10.8), and 2.3 in the INVS (95% CI: 0.8 to 6.5). The beneficial effects of argon laser treatment have persisted in all three studies in spite of the fact that some eyes in the "no treatment" groups were treated later in their clinical course as a result of the evidence of early benefit. Follow-up of all patients continues so that treatment effectiveness over a five-year period can be assessed.