Coats' disease diagnosed in adulthood is rare; therefore, the treatment options and longer clinical course are not well established. We report on two cases of adult onset Coats' disease, which have been observed for more than 10 years after conventional treatment. In the first case, a 76-year-old man with 9 years of diabetic retinopathy noticed a visual field defect in his left eye. Yellowish subretinal exudation with serous retinal detachment in his superior peripheral retina, and telangiectatic vessels with fluorescein leakage, numerous microaneurysms, and areas of capillary nonperfusion observed in a fluorescein angiography indicated adult Coats' disease, and retinal photocoagulation was applied. Within 1 year, subretinal exudation was regressed and visual acuity was improved from 20/50 to 20/20, and was maintained for the next 11 years. In the second case, a 71-year-old man presented with decreased vision in his right eye. The fundus of his right eye showed multiple telangiectasic vessels and subretinal exudates extended to the fovea, which is diagnosed as adult Coats' disease. Despite retinal photocoagulation, an increase of exudation and an enlargement of retinal detachment was observed within 1 month, and subsequently, additional treatment of cryotherapy was performed. Two months after these therapies, the exudation was regressed without retinal detachment, and visual acuity was improved to 20/200 which was maintained for the next 10 years. Even with adult Coats' disease, conventional therapies of retinal photocoagulation and cryotherapy are effective and are the initial choice for improving or maintaining visual function.