Pneumocystis pneumonia (PCP) is a serious opportunistic infection caused by a fungus named Pneumocystis jirovecii. A 20-year-old male patient came to the Emergency Department complaining about weakness, fatigue, achy muscles, joint pain, cold chills, fever, coughing, mucus production for about 2 weeks. Levofloxacin therapy was started with the diagnosis of community-acquired pneumonia to the patient who had no medical history of chronic disease or hospitalization.The patient was consulted as there was no clinical response. Physical examination revealed tachypnea, and ralles in both hemithorax were detected. No other abnormalities were identified on multi-systemic exam. Laboratory work-up showed leukocytes of 2800/cm (45% neutrophil), hemoglobin: 10.1 g/dL (htc: 33.5%), urea: 50 mg/dL, creatinine: 1.2 mg/dL, serum albumin level: 2.4 g/dL, LDH: 798 IU/L, elevated levels of C-reactive protein: 97 mg/L, procalcitonin: 7 ng/mL, sedimentation rate of 83 mm/hour and rapid HIV test was positive. HIV load observed was 125.000 copies/mL, CD4 + cell counts were 125/mm3. A CT scan revealed the presence of diffuse ground glass opacity. Lopinavir + ritonavir 2 x 2 tb + tenofovir disoproxil + emtricitabine treatment with 1 x 1 tb treatment was started. In addition, treatment with trimethoprim-sulfamethoxazole (TMP-SMX) 3 x 160 mg/kg IV and corticosteroids was initiated with the possible diagnosis of PCP pneumonia. Clinical and radiological findings improved and the patient’s treatment was completed in 21 days. PCP pneumonia is an important cause of pneumonia in people who have immunosuppressive diseases such as HIV/AIDS, or in people who are treated with immunosuppressive medications. Therefore, it is important to remember that HIV tests are also required in patients with risk factors or in the presence of clinical, radiological and laboratory findings consistent with opportunistic pneumonia.