Precursor B-lymphoblastic lymphoma (B-LBL) may present as a solitary bone tumor. Fewer than 10 cases with a proven precursor B-cell phenotype have been reported in the English literature. In this report, we describe four cases of B-lymphoblastic lymphoma presenting as a localized intraosseous mass, which clinically and histologically mimicked Ewing's sarcoma. Three tumors occurred in the tibia and one in the humerus. In all four cases, the initial diagnosis was either "Ewing's sarcoma" or "consistent with Ewing's sarcoma." All four patients were female. Three were children and one was an adult; mean age was 12.5 years (range, 4 to 31 years). All had extremity pain without significant constitutional symptoms. In three cases, the tumors were osteolytic on radiographic evaluation, and in one case, osteosclerotic. Immunohistochemical stains on paraffin-embedded tissue showed that the neoplastic cells expressed terminal deoxynucleotidyl transferase, CD43, vimentin, and CD99 (MIC2 gene product) in all cases. Three cases were negative for CD45. CD79a was positive in all four cases studied; however, CD20 (L26) was positive in only two of four cases. CD3 was negative in all cases. Two cases showed focal granular cytoplasmic staining for keratin. Two cases analyzed by polymerase chain reaction (PCR) revealed clonal rearrangement of the immunoglobulin heavy chain (IgH) gene. Follow-up revealed that the three pediatric patients, who received a high-dose multiagent chemotherapy regime for LBL, are disease free at follow-up intervals of more than 1, 11, and 12 years, respectively. The adult patient died two years after diagnosis with disseminated disease. Although rare, B-lymphoblastic lymphoma should be considered in the differential diagnosis of small round cell tumors of bone. A diagnosis of Ewing's sarcoma should be made only after complete immunophenotyping and, if necessary, molecular diagnostic tests to exclude lymphoblastic lymphoma. A limited panel of antibodies can lead to an erroneous diagnosis; B-lymphoblastic lymphoma may be negative for CD45 and CD20 but positive for CD99 and even for keratin, mimicking Ewing's sarcoma. Correct diagnosis is extremely important because LBL usually is curable in the pediatric age group with appropriate therapy.