The prevalence of secondary cancers associated with the breast cancer treatment has increased, which is due to the administration of cytotoxic/hormonal drugs as well as radiotherapy. A 54-year-old female patient with a history of breast cancer for 4 years and receiving tamoxifen the hematology clinic with fatigue and nosebleed. Laboratory parameters were revealed pancytopenia. The bone marrow biopsy finding was compatible with CD20 positive high-grade B cell lymphoma resembling diffuse large B cell lymphoma. The patient started to receive a chemotherapy. Her hemogram values displayed an improvement after the second cycle. However, interim PET-BT, performed after the fourth cycle, showed an incomplete response in cervical lymphatic nodes. Then, a tru-cut biopsy was performed resulting in breast cancer metastasis. This is an unusual case of secondary-DLBCL presenting with pancytopenia and occuring 4 years after the diagnosis of breast cancer. In conclusion, clinicians should carefully set the dosage of chemotherapy drugs to avoid the long-term side effects associated with such drugs.