Purpose: The aim of this paper is to describe a case of relapsed pediatric acute lymphoblastic leukemia (ALL) presenting as a rapidly progressive subretinal infiltrate, as diagnosed by ultrasound-guided fine needle aspiration (FNA). Methods: We conducted a clinical pathological retrospective chart review. Results: Eleven months after documented remission of T-cell ALL while on maintenance therapy, this 17-year-old patient presented with acute open angle glaucoma in the right eye. B-scan ultrasonography suggested total retinal detachment. Eight weeks later, based on routine cerebrospinal fluid analysis, the patient was diagnosed with central nervous system relapse of T-cell ALL. Repeat B-scan 1 week later showed a new hyperechoic subretinal mass. FNA of the mass confirmed leukemic infiltrate. The involved eye was enucleated, demonstrating leukemic cells throughout the subretinal space, choroid, and the optic nerve. Following hematopoietic stem cell transplant, the patient continues to maintain bone marrow remission 5 months after enucleation without involvement in the opposite eye. Conclusion: Retinal detachment in any patient with a history of leukemia should raise the possibility of relapse and may warrant aspiration/biopsy if other means of diagnosing relapse are inconclusive. Subretinal infiltrate may progress rapidly and prompt diagnosis is paramount to tailoring therapy and preserving vision.