Bone marrow necrosis, regarded as a rare finding in specimens from living patients, has been associated with a poor prognosis in patients with serious hematologic diseases and metastatic carcinoma. Two patients with extensive idiopathic bone marrow necrosis and a relatively benign course of illness were found. Therefore, we examined 500 consecutive bone marrow biopsy specimens that were obtained in a university hospital complex. Review of this material showed evidence of necrosis and degenerative changes of variable severity in one third of the biopsy specimens. It was found with approximately the same incidence in patients who underwent bone marrow biopsy for either neoplastic or nonneoplastic disorders; an increased prevalence was not observed in the group of patients who had received chemotherapy. Based on these observations, we believe necrosis and degeneration of the bone marrow is a commonplace finding that is frequently overlooked in a wide variety of acute and chronic disorders, and that requires further investigation to determine its clinical importance.