BACKGROUND: This study was initiated to test the hypothesis that soluble p185(HER-2) could be of value as a diagnostic or predictive marker for patients with malignant bone tumours. PROCEDURE: Sera of 35 patients with high-grade malignant osteosarcoma (n = 27) and Ewing Sarcoma (n = 8) were tested at the time of diagnosis by ELISA and compared with sera of controls (n = 38) and clinical data. RESULTS: In patients with osteosarcoma and Ewing Sarcoma, levels of sp185(HER-2) did not differ significantly from levels in controls. These results were irrespective of the type of tumour, survival chemotherapy or other clinical variables. CONCLUSION: p185(HER-2) serum levels do not appear to be of diagnostic or predictive value for differentiation of high-grade osteosarcoma and Ewing Sarcoma.