Retinal arterial thrombosis is rare in childhood. We describe a 15-year-old boy with sudden visual loss in his left eye. Ophthalmologic examination of the left eye showed an optic disk edema, and an upper visual field defect. Fluorescein angiography showed an occlusion of the lower temporal branch of the left retinal artery. Ocular motility and anterior segment structures were normal in both eyes. The boy’s medical history revealed hypercholesterolemia; cardiovascular disease and hypercholesterolemia were reported in the family. At admission, he was in good clinical condition. Physical examination revealed systolic ejection murmur. Procoagulant and anticoagulant factors (activated prothrombin time, partial thromboplastin time, free protein S, factor V Leiden, factor II, plasma homocystine, resistance to activated protein C, and antithrombin III) were normal. Hypercholesterolemia (282 mg/dL) and elevated level of lipoprotein (a) (1840 mg/dL, normal value <300 mg/dL) were detected. Chest x-ray, brain MRI, angiography, and carotid Doppler ultrasound were normal. A contrast echocardiography revealed a large right-to-left interatrial shunting across the patent foramen ovale. Anticoagulant (low weight heparin) therapy was administered. Two months later the child underwent surgical closure of his patent foramen ovale by using a transcatheter device, followed by anti-platelet acetylsalicylic acid therapy for 12 months. At 9 months follow-up was uneventful. Thus, in otherwise healthy children with isolated retinal arterial occlusion, a screening for underlying thrombophilia, including lipoprotein (a), should be performed. A paradoxical embolism, due to the presence of both foramen ovale patency and hypercoagulable state, could be hypothesized as the pathogenetic mechanism in our case.