A 47-year-old woman received adjuvant chemotherapy for breast cancer. On the 13th day of 4 courses of dose-dense AC therapy, she developed a fever. She was orally administered an antibioticfor febrile neutropenia treatment. She showed no improvement of symptoms and gradually presented with new symptoms, including a non-productive cough and dyspnea. After admission, she underwent a further examination, and was provided a diagnosis of pneumocystis pneumonia. It is reported that patients receiving chemotherapy for solid tumors are less likely to develop opportunistic infections. However, patients receiving dose-dense chemotherapy may have a higher risk for developing opportunistic infections than those receiving conventional chemotherapy.