The findings in bone marrow trephine biopsies correlate with the tumor burden in multiple myeloma. The pattern of plasma cell infiltration may be interstitial, focal, or diffuse. The extent of marrow involvement ranges from a small increase in the number of plasma cells to complete replacement by sheets of plasma cells. In early stages of the disease, the infiltrates are focal or interstitial and there is sparing of normal hematopoiesis. In advanced stages of myeloma, there is diffuse marrow involvement and hematopoiesis is suppressed resulting in cytopenias. Staging of Myeloma: Based on the extent of bone marrow involvement on trephine biopsies, a staging system has been proposed. Stage I: less than 20% of marrow replaced by neoplastic plasma cells; Stage II: 20%-50% involvement; Stage III: greater than 50% involvement. In this trephine biopsy of bone marrow from a myeloma patient, the plasma cells make up approximately 50% of the cells and the pattern of infiltration is interstitial. Image courtesy of: Genevieve Crane, MD, PhD, Johns Hopkins Hospital, Baltimore, Maryland, USA.