CD9 expression is not a prognostic factor in human osteosarcoma. CD 9, also known as Motility-Related Protein-1 (MRP-1), is a member of the transmembrane four superfamily and plays a crucial role in cell adhesion, motility and signalling events. Downregulation of CD 9 has been reported to be associated with tumour progression, metastasis and clinical outcome in various kinds of solid tumours. Although prognosis of osteosarcoma has been improved by chemotherapy during the last decades, the problem of non-responders remains. At the present time prognostic factors at diagnosis have not been clearly identified. Furthermore, there is a need for markers that predict the response to chemotherapy at the time of biopsy, allowing stratification of osteosarcoma patients. In this study we investigated the effect of CD9 expression on the response to chemotherapy and survival in osteosarcoma. The expression of CD9 was examined immunohistochemically in 52 patients with high grade osteosarcoma and the results were correlated with histologic response to chemotherapy, 5 year disease free and 5 year overall survival. In patients with osteosarcoma 22 of 52 cases (42%) were positive for CD 9 expression, the rest were negative. CD 9 expression status showed no statistically significant correlation with response to chemotherapy; 41% had a poor response and 59% a good response in the CD9 positive group. In the CD9 negative group 57% had a good and 47% had a bad response. No significant difference was found when comparing disease free survival (58.9% in CD9 positive- versus 69.3% in CD9 negative tumours; P = 0.99) and overall survival of patients (54.0% in CD9 positive- versus 58.1% in CD9 negative tumours; P = 0.90) with CD9 expressing tumours to those with reduced CD9 expression. In conclusion our findings suggest that in contrast to solid tumours, CD9 is unlikely to provide any additional prognostic information for clinical purposes in osteosarcoma patients.