A complete spontaneous regression of diffuse large B cell lymphoma involving the right breast, confirmed by aspiration cytology, is reported. The patient visited a hospital due to the rapid growth of a tumor in the right breast. Five years previously she underwent a craniotomy for a brain tumor, diagnosed as B‐cell malignant lymphoma, and received several courses of irradiation to the brain. Analysis of the breast tumor cells obtained by aspiration revealed lymphoma cells morphologically, which were similar to the tumor cells in the brain expressing CD20. While waiting for further examination, the tumor regressed rapidly and was not palpable after 20 days. An excisional biopsy of the breast exhibited no definite malignant lymphoma cells among a diffuse population of CD45RO and CD8‐positive small lymphocytes. Nucleotide sequencing of HCDR3s of the brain tumor and breast tumor cells showed a completely matched sequence, revealing the breast mass to be a metastatic lesion from the tumor of the brain. Although there was no tumorous lesion, the patient received additional chemotherapy and has shown no sign of recurrence in the breast for 7 years. We were able to confirm that the breast lymphoma shown in the aspiration cytology was a metastatic one, which was not proven histologically prior to chemotherapy, and regard the present case as a malignant lymphoma of the breast showing spontaneous regression. The present case shows a rare occurrence of spontaneous regression of diffuse large B cell malignant lymphoma after aspiration and suggests that CD8‐positive T cells might be related to the regression.