Objective To observe the preventive and therapeutic effect of different times, spot reactions and spot density of argon laser photocoagulation on retinal neovas cularization of ischemic retinal vein occlusion (IRVO).Meth9al of 244 patients (268 eyes) with IRVO diagnosed by fundus fluorescein angiography (FFA) were treated by HGM argon laser photocoagulator with green-blue light with 200~500 μm lightspot, 0.1~0.5 s, 0.3~1.0 w, and II~III class spot reaction . All capillary nonperfusion areas (CNA) were photocoagulated, and so were the retinal neovascularization in some patients. The follow up periods were from 6 to 60 months. After 3 and 24 weeks after photocoagulation FFA was performed again. Photocoagulation was performed supplementarilly for the new CNA or incompletely photocoagulated areas. Ophthalmoscopic examination and FFA were performed in all the patients after half a year.Results Only 17 eyes (10.6%) with neovascularization were found after preventive photocoagulation in 160 eyes in non-neovacularization group. Sixty-nine eyes(63.9%) with neovascular atrophy and 39 eyes (36 .1%) with unsuccessful photocoagulation were found after therapeutic photocoagulation in 108 eyes in neovascularization group. There was statistical significance between the two groups (P＜0.01). Photocoagulation energy with reaction of III class and density of 1 lightspot diameter was more effective than which with reaction of II~III class and density of 1.5 lightspot diameter or reaction of ≤II class and density of 2 lightspot diameter (P＜0.01). Conclusion Efficacy of preventive photocoagulation is better than which of therapeutic photocoagulation. Photocoagulation energy with reaction of III class and density of 1 lightspot diameter is an effective method for IRVO.(Chin J Ocul Fundus Dis，2003，19：201-268)