AbstractClosed trephine needle biopsy of the bone marrow has become an established procedure in the evaluation of many malignant and benign diseases in adults; however, its role in pediatric pathology has not yet been defined.In the period from February 1974 to April 1978 we have performed 164 such biopsies in 111 children under 15 years of age. A representative specimen has been obtained in over 80% of cases. This series included, in order of frequency, non‐Hodgkin lymphomas, Hodgkin lymphomas, aplastic anemias, rhabdomyosarcomas, neuroblastomas, miscellaneous solid tumors, and single cases of histiocytosis X, malignant histiocytosis, sarcoidosis, malignant histiocytoma, and Castleman lymphoma of the hyaline‐vascular type.Histology has been found superior to cytology in the detection of neuroblastoma invasion; the evaluation of the true cellularity in aplastic anemia, and the detection of granulomatous tissue in the only case of sarcoidosis. In other diseases histology and cytology gave similar information, except for the few cases of acute leukemia in partial relapse, which has been better defined in the aspirate smears than in the core specimen.Further evaluation of this technique in other patient series appears advisa