Pneumocystis pneumonia(PCP) is a life-threatening infection in immunocompromised patients. It is relatively uncommon in patients with lung cancer. We report a case of PCP in a 59-year-old man with a past medical history of chronic obstructive pulmonary disease treated with formoterol and a moderate daily dose of inhaled budesonide. He had also advanced stage non-small lung cancer treated with concurrent chemo-radiation with a cisplatin-etoposide containing regimen. The diagnosis of PCP was suspected based on the context of rapidly increasing dyspnea, lymphopenia and the imaging findings. Polymerase chain reaction testing on an induced sputum specimen was positive for Pneumocystis jirovecii. The patient was treated with oral trimethoprim-sulfamethoxazole and systemic corticotherapy and had showed clinical and radiological improvement. Six months after the PCP diagnosis, he developed a malignant pleural effusion and expired on hospice care. Through this case, we remind the importance of screening for PCP in lung cancer patients under chemotherapeutic regimens and with increasing dyspnea. In addition, we alert to the fact that long-term inhaled corticosteroids may be a risk factor for PCP in patients with lung cancer. Despite intensive treatment, the mortality of PCP remains high, hence the importance of chemoprophylaxis should be considered. Pneumocystis pneumonia (PCP) is a life-threatening infection in immunocompromised patients. It is more uncommon in patients with lung cancer. We report a case of PCP in a 59-year-old man with a past medical history of chronic obstructive pulmonary disease treated with formoterol and a moderate daily dose of inhaled budesonide. He had also advanced stage non-small lung cancer treated with concurrent chemo-radiation with a cisplatin-etoposide containing regimen. The diagnosis of PCP was suspected based on the context of rapidly increasing dyspnea, Lymphopenia and the imaging findings. an Polymerase chain reaction testing on an induced sputum specimen was positive for Pneumocystis jirovecii. The patient was treated with oral trimethoprim-sulfamethoxazole and systemic corticotherapy and had showed clinical and radiological improvement. Six months after the PCP diagnosis, developed a malignant pleural effusion and expired on hospice care. Through this case, we remind the importance of screening for PCP in lung cancer patients under chemotherapeutic regimens and with increasing dyspnea. In addition, we alert to the fact that long-term inhaled corticosteroids may be a risk factor for PCP in patients with lung cancer. Despite intensive treatment, the mortality of PCP remains high, hence the importance of chemoprophylaxis should be considered.