Tumor lysis syndrome is an oncologic emergency that usually occurs after chemotherapy in patients with hematologic malignancies. Tumor lysis syndrome is rare in cases of solid tumors, especially when it occurs spontaneously. Herein, we present a case of spontaneous tumor lysis syndrome in a 55-year-old woman who presented with dyspnea and was found to have extensive metastatic small cell lung cancer. She developed acute oliguric renal failure and multiple electrolyte abnormalities requiring hemodialysis. The findings of this case suggest that clinicians should maintain a high index of suspicion for patients with malignancies who demonstrate the classic symptom of laboratory abnormalities even in the absence of chemotherapy.