A girl in her teens with a history of acute lymphoblastic leukemia, diagnosed 7 years before presentation and treated with chemotherapy, presented with 5 days of complete vision loss in the left eye. On examination, visual acuity was 20/20 OD and no light perception OS with a significant afferent pupillary defect. The anterior segment was normal. On fundus examination, the right eye was normal; the left eye showed significant anteriorization, diffuse swelling, and hemorrhage of the optic nerve, 360° of intraretinal hemorrhage, and retinal whitening and edema along the inferior arcade (Figure). Magnetic resonance imaging showed diffuse thickening of the left optic nerve without orbital mass. She was diagnosed with acute lymphoblastic leukemia relapse to the optic nerve causing a secondary retinal vein occlusion and hemiretinal artery occlusion. She was treated emergently with extended beam radiation therapy at 1800 cGy in 10 treatments.