BACKGROUND Monocarboxylate transporter 4 (MCT4) is a critical element for glycolytic metabolism and malignant behaviors in many tumor cells. This study aimed to determine the expression level of MCT4 protein and its prognostic value in osteosarcoma. MATERIAL AND METHODS MCT4 expression was detected via immunohistochemical and Western blot analysis for 100 osteosarcoma patients. The correlation between MCT4 expression and clinical factors among the patients was analyzed using the chi-square test. Overall survival of osteosarcoma patients was estimated by Kaplan-Meier analysis. The prognostic value of MCT4 was evaluated using Cox regression analysis with adjustments for clinicopathological variables. RESULTS MCT4 expression was significantly upregulated in osteosarcoma tissues compared with that in adjacent normal ones, detected via both immunohistochemical and Western blot analyses. High MCT4 expression showed a positive association with distant metastasis (P=0.000) and recurrence (P=0.000) of osteosarcoma. Kaplan-Meier analysis indicated that overall survival of osteosarcoma patients was significantly higher in the low MCT4 expression group than in the high expression group (log-rank test, P<0.001). Multivariate analysis indicated that MCT4 expression and clinical stage, which are tightly related to the prognosis of osteosarcoma, might be independent predictors of osteosarcoma prognosis. CONCLUSIONS High MCT4 expression appears to contribute to osteosarcoma progression and the upregulation of MCT4 may predict poor prognosis among osteosarcoma patients.