Pneumocystis jirovecii (formerly carinii) pneumonia (PJP) is an opportunistic infection well-recognized in patients with profound T cell immunodeficiency. It is much less common in patients with solid tumors unless they have other major predisposing factors such as prolonged treatment with corticosteroids or T4 lymphocyte counts of less than 200 cells/mm3. We present a previously unreported case of fatal PJP in a breast cancer patient with bone metastases who was receiving a first-line treatment with weekly paclitaxel, trastuzumab, and dexamethasone as premedication for paclitaxel. She had received eight doses of paclitaxel at 80 mg/m2, trastuzumab 2 mg/m2, and dexamethasone 10 mg for just over 7 weeks when she was diagnosed with PJP. While the patient’s granulocyte counts were normal throughout her treatment, the total lymphocyte counts reached the nadir of 400 cells/mm3 a few days after the eighth dose of chemotherapy – around the time of PJP diagnosis. Both dexamethasone and the total lymphocyte nadir predisposed this patient to PJP.