Differential cell counts were performed on 200 paired bone marrow aspirates and trephine biopsy specimens to compare the distribution of cell types. Relatively more immature myeloid cells were found in the trephine biopsy specimens and relatively more polymorphs and lymphocytes in the aspirates. Two methods for sampling areas of the trephine biopsy specimens for counting were assessed, and the differences between aspirates and trephine specimens were found to be more consistent when the second, more extensive, sampling method was used. This method also permitted quantitation of some features of bone marrow topography and provided information that would not normally be obtainable from aspirated material. The techniques were easy to apply and took relatively little time to perform. They could offer useful information in the study of bone marrow disorders, particularly those such as myelodysplastic syndromes in which disturbances of marrow architecture are prominent.