Aims: To assess the value of measuring the subpopulations of T and B lymphocytes in patients with musculoskeletal tumours as an immunodiagnostic procedure in primary diagnosis of tumours. Methods: In this prospective study blood samples were obtained from 145 patients aged 04–98 presenting with musculoskeletal tumours. Measurements of subpopulations of T and B lymphocytes (CD3, CD4, CD8, Helper/Suppressor ratio and CD19) were based upon the principle of Ortho Cytoron Absolute Flow Cytometry. The histological diagnosis of tumours was obtained by the histopathological investigation of the biopsy samples and the ßow cytometry results allocated accordingly. Results: Of the 145 patients, osteomyelitis was diagnosed in 15 (10.34%) patients. Median values of subpopulations of T and B lymphocytes were signiþcantly raised in patients with osteomyelitis, as follows: CD3 2456, CD4 1479, CD8 929, Help/Sup ratio 1.8 and CD19 560. Results were also calculated for osteosarcomas, Ewing sarcomas, giant cell tumours, chondrosarcomas, metastatic carcinomas, synovial cell sarcomas, chondroblastomas and others, with their median values within normal reference levels. To conþdently out rule or conþrm the diagnosis of osteomyelitis the cut off point values of lymphocytes (the highest value in other patients groups) were measured. The cut off point values were found to be: CD3 2420, CD4 1400, CD8 873 and CD19 550. Conclusions: The subpopulattions of T and B lymphocytes were signiþcantly raised in patients with osteomyelitis in contrast to other causes. In this study it is clearly shown that the levels of CD3, CD4, CD8 and CD19 above the presented cut off values are an important and accurate conþrming factor for the diagnosis of suspected osteomyelitis.