Previous studies with animal tumors showed that bone marrow (BM) is a privileged site where potentially lethal tumor cells are controlled in a dormant state by the immune system. Here, we investigated BM of breast cancer patients with respect to tumor cell content, immune activation status and memory T‐cell content. BM‐derived cells from primary operated breast cancer patients (n = 90) were compared with those from healthy donors (n = 10) and also with cells from respective blood samples. Cytokeratin 19‐positive tumor cells were detected by nested polymerase chain reaction. Three‐color flow cytometry was used to identify numbers and activation state of T cells, natural killer (NK) cells, monocytes/macrophages and subsets by a panel of monoclonal antibodies (mAbs). The proportion of memory T cells among the CD4 and CD8 T cells was much higher in BM of cancer patients than in healthy donors (p < 0.001). The extent of memory T‐cell increase was related to the size of the primary tumor. Patient‐derived BM memory CD8 T cells could be shown to contain specific HLA‐A2/Her‐2/neu_369–377 tetramer binding cells. Patients with disseminated tumor cells in their BM had more memory CD4 T cells and more CD56⁺ CD8⁺ cells than patients with tumor cell‐negative BM. Only some of the immunological changes seen in BM samples of cancer patients were also detectable in peripheral blood samples. Our hypothesis that BM is a special compartment for immunological memory and tumor dormancy is supported by the above findings. The overall results reveal that BM is a valuable additional compartment for immune diagnosis in pathological conditions and possibly for follow‐up treatment strategies. © 2001 Wiley‐Liss, Inc.