Poorly differentiated non-small cell lung carcinoma presenting as a fever of unknown origin.

To achieve a definitive diagnosis of a fever of unknown origin (FUO), clinicians must consider infectious, autoimmune, neoplastic and other pathologies. While thorough history taking is crucial, it may not reveal the underlying aetiology behind the FUO. Several weeks of laboratory testing, imaging studies and advanced testing may be involved. We present the case of a woman in her 70s who came to her primary care provider with a primary complaint of FUO and describe the diagnostic steps, which identified a poorly differentiated, advanced non-small cell lung carcinoma. This case also highlights several missed diagnostic opportunities, including delayed imaging and diagnostic inertia, which contributed to the late identification of her underlying malignancy.