We histologically and immunohistochemically studied 37 cases of diffuse large B-cell lymphoma (DLBCL) initially manifesting in the bone marrow (BM). We also compared these cases with the Asian variant of intravascular large B-cell lymphoma (AIVL). Histologically, the neoplastic cells of the BM mostly had large and round nuclei and formed clusters. Immunohistochemically, all cases were positive for B-cell markers. Factor VIII staining revealed neoplastic cells within the sinusoids of BM in 8 cases; however, these cells accounted for fewer than 20% of the overall neoplastic cells. In several cases, the neoplastic cells infiltrated liver, spleen, kidneys, lungs, stomach, and adrenal glands with a mainly leukemic and infrequently intravascular pattern. Although our cases share some clinical features with AIVL, we consider DLBCL initially manifesting in the BM to be a unique entity because the neoplastic cells proliferate mainly in BM, with infrequent involvement of the sinusoids and occasional leukemic infiltration in various organs.