Tumor lysis syndrome (TLS) is an oncologic emergency characterized by the destruction of tumor cells leading to an influx of large amounts of uric acid, potassium, and phosphorus into systemic circulation. It most often occurs after the initiation of cytotoxic therapy in high-grade lymphomas and leukemias; however, rarely it may occur spontaneously. The authors report a case of spontaneous tumor lysis causing electrolyte abnormalities and acute kidney injury in a patient with subsequently diagnosed large chronic lymphocytic leukemia tumor burden. Spontaneous TLS can be the first presentation of underlying malignancy; therefore, physicians should be aware of the associated findings.