Primary bone marrow lymphomata are infrequent; most of them are of B-cell origin, and those of a T-cell lineage produce mainly both hypercalcemia and osteolytic lesions apparently due to abnormal production of osteoclast-activating factor. We report a 15-year old patient with a primary bone marrow lymphoma: 85% of his infiltrating malignant lymphocytes displayed cytoplasmic mu-chains compatible with a pre-B phenotype. The cells failed to display the CALLA/CD 10 antigen. Serum calcium was 7.5 mEq/L (range 4-5 mEq/L); the bone biopsy of an osteolytic lesion disclosed a large-cell, diffuse non-Hodgkin's lymphoma. No malignant cells were found in the peripheral blood and there were no enlarged lymph nodes. The patient was treated with 6 courses of chemotherapy: hydroxyldaunorubicin, vincristine and prednisone (HOP). Complete remission was achieved and the patient was placed on continuation chemotherapy with daily six-mercaptopurine and weekly methotrexate, together with HOP pulses every three months. The hypercalcemia disappeared together with the fever and the bone pain: the patient has been followed 6 months. Data on this case are discussed together with those previously published in regard to the low prevalence of bone lesions in primary B-cell lymphomas of the bone marrow, and to the similarity of this B-cell malignancy to others that produce both hypercalcemia and bone lesions, i.e. multiple myeloma.