We undertook this study to evaluate the efficacy of bone marrow aspiration as compared with trephine needle biopsy. In the 50 consecutive cases reviewed, simultaneous aspiration and biopsy yielded significant morphologic findings in either or both. A comparison was made between the morphologic findings in the aspirate and the corresponding biopsy, with quantification of these findings when feasible. The aspiration technic and method of processing used differ significantly from the so-called clot preparation currently in widespread use. For coring the marrow, a regular (adult) 11 gauge Jamshidi needle was used. More lymphoid nodules, nodular lymphoid hyperplasia, and lipid granulomas were found in the aspirate.