The Idiopathic Neovascularization Study--Krypton Laser is a multicenter randomized controlled clinical trial conducted to determine whether krypton red laser photocoagulation is useful in preventing or delaying loss of visual acuity in eyes that have either choroidal neovascularization 1 to 199 microns from the center of the foveal avascular zone or choroidal neovascularization 200 microns or farther from the foveal avascular zone center with associated blood and/or blocked fluorescence extending within 200 microns of the foveal avascular zone center in the absence of any other clinically significant eye disease. Eligible lesions could have blood or blocked fluorescence that extended through the entire avascular zone. Patients were assigned to laser photocoagulation (n = 24) or to observation only (n = 25). With 65% of these patients followed up for 5 years and 85% for 3 years, there is evidence that krypton laser treatment is beneficial. After 3 years of follow-up, 2 (10%) of 20 treated eyes in comparison with 7 (37%) of 19 untreated eyes had lost six or more lines of visual acuity (P = .15). Furthermore, more untreated eyes than treated eyes had experienced such losses at each point in follow-up after the 3-month examination. The findings from this trial taken with findings of other trials conducted by the same investigators for lesions similar in appearance and location but secondary either to age-related macular degeneration or to ocular histoplasmosis support the conclusion that patients with idiopathic parafoveal neovascularization benefit from laser treatment.