The role of bone marrow biopsy is expanding. Bone marrow biopsy is considered a "gold standard" for assessing cellularity and infiltrative process. More recently, biopsies are increasingly used for immunohistochemical stains and molecular studies such as in situ hybridization and laser microdissection, further augmenting morphologic interpretation. Biopsies are playing a greater role in early diagnosis of myelodysplastic syndromes (MDS). Both primary and secondary MDS are increasing (5% to 12%), and can present a significant challenge in early diagnosis. Present MDS diagnostic criteria are based purely on marrow aspirate smears, peripheral blood smears, and ancillary studies, and may not be adequate in some instances. With the current understanding of marrow topography and hematopoietic microenvironment, bone marrow biopsies are greatly useful in early diagnosis and grading of MDS. Marrow biopsies also are helpful in evaluation of postchemotherapy and post-bone marrow transplant patients. Assessment of topographic alteration and certain corroborating immunohistochemical and molecular studies can only be performed on bone marrow biopsies, and these ancillary studies will contribute significantly to the understanding of hematopoietic disorders. Bone marrow biopsy, in conjunction with other modalities such as flow cytometry, will play a significant role in diagnostic hematopathology. Copyright © 2001 by W.B. Saunders Company.