Bone marrow examination is not necessary for the diagnosis of CLL. However, it may offer useful information regarding the status of hematopoiesis. Lymphocytes make up more than 30% of all cells in the bone marrow aspirate smears. Their morphology is similar to that of circulating leukemic cells. The bone marrow core biopsies show different patterns of infiltration: a) diffuse, solid lesions which appear dark on low magnification; the least common pattern, b) focal, non-paratrabecular nodules, and c) interstitial pattern in which the CLL cells are admixed with other hematopoietic elements. Proliferation foci and true germinal centers are extremely rare. Extensive solid, diffuse infiltration of the bone marrow is associated with cytopenias, advanced clinical stage, and has been linked with the presence of 70-kDa zeta-associated protein (ZAP-70). The image shows high magnification of a bone marrow aspirate smear. The predominant cell population is small, dark typical cells of CLL with scanty cytoplasm and condensed nuclear chromatin. Occasional megakaryocytes are also present.