Diffuse large B-cell lymphoma (DLBCL) is well known for selectively involving certain extranodal locations such as the central nervous system (CNS), testes, and skin. DLBCL or high-grade B-cell lymphoma selectively involving the bone marrow is rare and has been sparsely reported in the medical literature. We report two cases of lymphoma presenting with primary bone marrow involvement without evidence of involvement of any other sites. The first case represents de novo DLBCL. The patient achieved complete remission with initial treatment, had a bone marrow-only relapse three years later, and achieved a second complete remission following non-transplant salvage therapy. The second case had findings consistent with "double hit" Richter's transformation of chronic lymphocytic leukemia with translocation of c-MYC and BCL-2. This patient had an aggressive clinical course characterized by rapid progression with CNS involvement within three months resulting in the demise of the patient. These two cases represent two distinct subtypes of primary bone marrow lymphoma: de novo and transformed. Further research is necessary to gain a better understanding of this rare lymphoma entity and develop novel therapies.