Hematologic oncology patients are particularly susceptible to infections due to immunosuppression caused by both the cancer itself and the treatment used. This patient group often experiences infectious complications of bacterial, viral, and fungal etiology. One of the pathogens responsible for pneumonia, especially in individuals with profound immunosuppression, is Pneumocystis jirovecii (PCJ), a fungus that produces cysts detectable in bronchial washings or sputum. This infection most commonly affects patients with HIV; however, it can also occur in individuals with hematologic diseases treated with the VTD regimen (bortezomib, thalidomide, and dexamethasone), who achieved partial response (PR) and subsequently underwent autologous hematopoietic cell transplantation (auto-HCT) as consolidation therapy. The transplantation was uneventful; however, during post-chemotherapy bone marrow aplasia, the patient developed febrile neutropenia. Empiric broad-spectrum antibiotic therapy was initiated. Blood cultures were negative, but a sputum sample revealed PCJ cysts. Despite initial respiratory colonization status, therapeutic doses of trimethoprim-sulfamethoxazole (TMP-SMX) were administered due to persistent cough and high risk of severe complications. Early detection of cysts and therapeutic TMP-SMX administration prevented full-blown PCJ pneumonia , underscoring prophylaxis effectiveness in severely immunocompromised patients.