Experiments show that collaterals that develop and maturate in branch retinal vein occlusion (BRVO) are helpful in the drainage of blood from a blocked area to an adjacent area. Laser treatment on the nonperfusion area can reduce the retinal blood inflow that can impair the formation of collateral vessels. Furthermore, if collaterals were accidentally destroyed by laser photocoagulation, leakage and neovascularization would increase. Forty-five patients with BRVO were reviewed retrospectively. Collateral vessels were noted from angiographic analysis in 27 of 45 (60%) patients. Good visual prognosis was noted in the patients with collaterals. Cases with neovascularization underwent laser treatment. In one case, laser photocoagulation was applied to the collateral vessels accidentally, after which the leakage significantly increased on fluorescein angiography. In conclusion, collateral vessels in BRVO have a favorable effect on visual prognosis. Careful laser treatment is recommended to avoid destroying collaterals in BRVO.