Twenty-eight cases of myelodysplastic syndrome (MDS) were reviewed to evaluate whether the morphologic criteria proposed by the French-American-British (FAB) Cooperative Group for marrow smears could be applied to glycol methacrylate embedded trephine biopsies. Bone marrow biopsies and marrow smears were examined separately and then compared for the following parameters: percentage of blasts, dyserythropoiesis, ring sideroblasts, dysmegakaryopoiesis, dysgranulopoiesis, monocytes, cellularity, fibrosis, and "abnormal localization of immature precursors". The results of the histologic (biopsies) and cytologic (marrow smears) examinations were in good agreement in 24 of 28 cases. The authors' results suggest that the five MDS types proposed by the FAB group can be reliably distinguished on bone marrow biopsy with knowledge of the peripheral blood blast and monocyte counts. When the bone marrow aspiration is inadequate, the biopsy can establish diagnosis and type of MDS and rules out aplasia or tumor infiltration as possible alternative causes of cytopenia.