To investigate the retinal vascular abnormalities in both affected and fellow eyes of presumed unilateral Coats' disease patients using ultra-wide-field fluorescein angiography (UWF-FA) and their association with visual prognosis. A retrospective review of 30 patients diagnosed with presumed unilateral Coats' disease, evaluated with UWF-FA from March 2003 to May 2024 in a tertiary referral hospital. Clinical features and multimodal imaging findings were evaluated and factors related to final visual outcomes were analyzed. All 30 patients were diagnosed with presumed unilateral Coats' disease at presentation, comprising 36.7% childhood-onset and 63.3% adult-onset patients. Retinal vascular telangiectasia was observed in 51.7% of fellow eyes. The extent of telangiectasia and exudate in the affected eyes did not significantly correlate with extent of telangiectasia in the fellow eyes. In the more affected eyes, the childhood-onset group had a significantly greater extent of capillary drop-out compared to the adult-onset group (5.0 vs 2.8 clock hours, p = 0.023). In the fellow eyes, telangiectasia tended to be more frequent in the childhood-onset group (63.6% vs. 44.4%), without significance (p = 0.160). In the multivariable regression analysis, the final best-corrected visual acuity (BCVA) in the more affected eye was significantly associated with initial BCVA. The mean extent of telangiectasia in the temporal and nasal quadrants and the initial BCVA of the more affected eyes were identified as prognostic factors for final moderate to severe visual loss (Snellen BCVA < 20/66) (OR = 12.8, p = 0.043; OR = 11.8, p = 0.024, respectively). About half of presumed unilateral Coats' disease cases exhibited features of bilateral disease. Visual prognosis is associated with the peripheral retinal telangiectasia in the temporal and nasal quadrants as well as initial BCVA in the affected eyes while it is not associated with retinal vascular abnormalities in the fellow eyes.