With completion of follow-up of all patients enrolled in three randomized clinical trials of argon laser photocoagulation of extrafoveal choroidal neovascular membranes secondary to senile (age-related) macular degeneration, ocular histoplasmosis, or idiopathic causes, the Macular Photocoagulation Study Group has demonstrated that laser treatment of such lesions is beneficial in preventing or delaying large losses of visual acuity for at least 5 years. In eyes with senile (age-related) macular degeneration as the underlying cause, the relative risk of losing six or more lines of visual acuity from the baseline level among untreated eyes (n = 117) compared with laser-treated eyes (n = 119) was 1.5 from 6 months through 5 years after entry (P = .001). In addition, after 5 years, untreated eyes had lost a mean of 7.1 lines of visual acuity, while laser-treated eyes had lost 5.2 lines. Recurrent neovascularization had been observed in 54% of laser-treated eyes by the end of the 5-year follow-up period. Among eyes with ocular histoplasmosis, untreated eyes (n = 130) had 3.6 times the risk of laser-treated eyes (n = 132) of losing six or more lines of visual acuity (P less than .0001). Also, untreated eyes had lost a mean of 4.4 lines of visual acuity after 5 years, compared with only 0.9 lines lost by laser-treated eyes. Among laser-treated eyes, recurrent neovascularization had been observed in 26% by 5 years after enrollment.(ABSTRACT TRUNCATED AT 250 WORDS)