Current treatment of high-grade osteosarcoma combines surgical removal of the lesion with chemotherapy. In this study we evaluated whether the expression of P-glycoprotein, a protein closely associated with multidrug resistance, may be helpful in identifying the patients whose tumours will be further resistant to specific agents. By using multidrug-resistant osteosarcoma cell lines as standards, the expression of P-glycoprotein was evaluated in 105 cases of primary and metastatic osteosarcoma by semiquantitative immunofluorescence. Overexpression of the protein was shown in 23% of primary and in 50% of metastatic lesions. In 38 cases, homogeneously treated and followed-up for at least 24 months, overexpression of P-glycoprotein appeared to be associated with a higher relapse rate and with a trend toward a worse outcome. These data support the role of P-glycoprotein in the response to chemotherapy and its involvement in the determination of the outcome of osteosarcoma patients.