Bone marrow involvement is often observed in patients with hematological malignancies and solid tumors. Sensitive immunological and molecular biological methods allow the detection of isolated transformed cells in bone marrow samples. Like normal hematopoietic cells, tumor cells interact with bone marrow stroma through adhesion molecules. Since adhesion molecules play an essential role in the invasion and proliferation of malignant cells, new therapeutic approaches involving the inhibition of cell-cell and cell-matrix interactions are conceivable.