Tumor lysis syndrome (TLS) is a medical emergency that can develop in leukemias and lymphomas as a first presentation or after the initiation of anti-neoplastic regimens. On the other hand, tumor genesis syndrome (TGS) is a rare condition associated with certain malignancies, especially those with a high neoplastic burden characterized by rapid proliferation, leading to avid uptake of phosphorus from the serum and culminating in hypophosphatemia. Interestingly, a combination of TLS and TGS can occur simultaneously in a subset of patients. This leads to the development of hypophosphatemia instead of the hyperphosphatemia commonly associated with TLS.We herein present a case of severe asymptomatic hypophosphatemia in a patient with an incidental finding of T-cell acute lymphoblastic leukemia. The patient was initially diagnosed with TLS with hypophosphatemia, but further investigation revealed that the patient had isolated TGS.