BACKGROUND In recent years, there has been an increasing incidence of Pneumocystis jirovecci pneumonia(PCP)in immunosuppressed non-HIV patients. However, only a few studies on PCP developed during chemotherapy for gastrointestinal cancer have been reported. Case 1: A 72-year-old man was complaining of dyspnea during chemotherapy for unresectable gastric cancer. The patient showed high β-D-glucan levels, and his sputum tested positive for sputum Pneumocystis PCR. Even after TMP-SMX administration, the patient's respiratory condition worsened; hence, intubation was needed. Finally, he died without showing any improvement. Case 2: A 75-year-old man underwent chemotherapy for a recurrence of cecal cancer and received steroid pulse for adverse events of optic neuritis. However, his respiratory condition worsened. Furthermore, his sputum tested positive for Pneumocystis PCR. Intensive care including TMP-SMX administration followed to improve his condition. CONCLUSIONS PCP with non-HIV has a more acute onset and a poorer prognosis than that with HIV. It is necessary to identify PCP when there is a rapid progression of respiratory symptoms and pneumonia in cancer patients undergoing chemotherapy or steroid treatment.