We describe a case of Pneumocystis carinii pneumonia (PCP) in a woman with recurrent breast cancer without human immunodeficiency virus (HIV) infection. The PCP was associated with severe lymphocytopenia due to treatment with anticancer agents in combination with granulocyte colony-stimulating factor (G-CSF). Despite the severe lymphocytopenia, the total leucocyte count never fell below 3000/mm3 during the treatment. It was difficult to determine whether the patient's respiratory failure was caused by severe infectious pneumonia, hypersensitivity pneumonia or pneumonitis carcinomatosis. She was treated with steroid for suspected drug-induced hypersensitivity pneumonia. However, as her condition did not improve, PCP was suspected, and sulfamethoxazole-trimethoprim was administered. At the same time, anticancer drugs were administered to half the progression of the cancer, since lymphangitis carcinomatosa was also suspected. The severe respiratory failure did not improve, and the patient died on day 23 after admission. At autopsy, the cause of death was confirmed to respiratory failure due to PCP.