A 42-year-old woman was admitted to the National Cancer Center Hospital in Tokyo, Japan, because of lumbago and bilateral leg pain. Clinical examination revealed a retroperitoneal bulky tumor, a breast tumor, and lymphoblasts in bone marrow. She did not have surface lymph node swelling or systemic symptoms such as weight loss and night sweats. Lymphoblasts in bone marrow were positive for CD10 and CD19 but negative for CD20, surface immunoglobulin, and T-cell antigens. Needle biopsy of a retroperitoneal mass revealed diffuse infiltration of lymphoblastic tumor cells. Because the cells were immunoreactive for CD79a, CD10, and terminal deoxynucleotidyl transferase, the patient was diagnosed as having precursor B-lymphoblastic lymphoma (which is rare in adults) with bone marrow involvement. The patient achieved complete remission by an induction therapy for acute lymphoblastic leukemia, underwent allogeneic bone marrow transplantation, and has remained in complete remission for more than 3 years.