Objective: The aims of our study are to evaluate the histologic features of metastatic neuroblastoma in bone marrow (BM) in comparison with those of primary neuroblastoma, and to compare the histologic characteristics of metastatic neuroblastoma in BM before vs. after chemotherapy. Method: Total 92 biopsies from 19 children diagnosed as stage 4 neuroblastoma with BM metastasis were examined histologically; 19 primary neoplasm biopsies, 19 BM biopsies before chemotherapy, 19 primary neoplasm excision biopsies after chemotherapy, and 35 BM biopsies after chemotherapy. Results: 1) Primary neoplasms were classified as neuroblastoma poorly differentiated (n=10), neuroblastoma differentiating (n=5), ganglioneuroblastoma intermixed (n=1) and neuroblastoma type unclassifiable (n=3). 2) Metastatic foci in BM before chemotherapy were composed of undifferentiated and/or differentiating neuroblasts but not ganglion cells, in neuropil but not schwannian stroma. 3) Metastatic foci of BM after chemotherapy showed differentiation such as ganglion cells and schwannian stroma, which was more prominent after more cycles of chemotherapy. 4) Metastatic neuroblastomas in BM after chemotherapy were as mature as or less mature than those in primary neuroblastomas after chemotherapy. Conclusion: Metastatic neuroblastomas in BM initially consist of more immature components than primary neuroblastomas, whereas they become differentiated as primary neuroblastomas after multi-cycle chemotherapy.