A 64-year-old male presented to hospital with breathlessness and weight loss. Ultrasound-guided biopsy of supraclavicular lymph node confirmed a diagnosis of small-cell lung cancer. The patient was started on Dexamethasone 8 mg twice daily for symptom control while awaiting urgent oncology assessment. Three days later he was admitted with acute kidney injury and worsening breathlessness. Biochemical changes confirmed tumour lysis syndrome (TLS) that had occurred following steroid therapy. He was given allopurinol followed by rasburicase. His clinical condition continued to worsen and he died of multi-organ failure. To our knowledge, TLS in small-cell lung cancer solely attributed to steroid therapy has not been described before. Due to its rarity, physicians have a very low index of suspicion of TLS in lung cancer when prescribing corticosteroids for palliation of symptoms. Patients with risk factors should be identified and baseline blood tests performed and appropriate prophylaxis commenced.