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A man in his 80s with small cell lung cancer (SCLC) was admitted to hospital with suspected neutropaenic sepsis after a single cycle of chemotherapy. He failed to improve with broad-spectrum antibiotics and was noted to be lymphopaenic and recurrently hypokalaemic. Serum cortisol level was markedly elevated, suggesting SCLC-related paraneoplastic Ectopic Cushing’s Syndrome (ECS). Induced sputum revealed Pneumocystis jirovecii. Despite high-dose co-trimoxazole, he died from respiratory failure due to progressive multilobar pneumonia. We suggest that immunosuppression due to occult SCLC-related hypercortisolaemia resulted in death from P. jirovecii pneumoniae infection and that ECS should be considered as a cause of hypokalaemia and lymphopaenia in patients with SCLC.