Nuclear expression of YB-1 protein correlates with P-glycoprotein expression in human osteosarcoma. The Y-box-binding protein, YB-1, is a member of the DNA-binding protein family. It binds to the Y-box, an inverted CCAAT box, in the promoter region of the human multidrug resistance 1 gene, which encodes P-glycoprotein (P-gp). Nuclear localization of YB-1 protein has been reported to be associated with the intrinsic expression of P-gp in human breast cancer. We studied the immunohistochemical expression of YB-1 protein in 69 untreated biopsy specimens of conventional osteosarcomas and compared it with the expression of P-gp. Furthermore, cell proliferation, as determined by the MIB-1-labeling index (MIB-1-LI), was measured by immunohistochemistry. In all 69 untreated osteosarcomas, YB-1 protein was expressed in the cytoplasm. In 32 of 69 (46%) cases, YB-1 was also localized in the nucleus. The expression of P-gp was evident in 23 of these 32 cases, and there was a significant correlation between the nuclear expression of YB-1 and P-gp expression (P < 0.0001). Chondroblastic osteosarcoma expressed YB-1 in the nucleus more frequently (eight of nine cases) than did other types of osteosarcoma, whereas P-gp was also frequently expressed in chondroblastic subtype. There was no correlation between the nuclear expression of YB-1 and histological grade. The MIB-1-LI was significantly higher in cases showing the nuclear expression of YB-1 (MIB-1-LI averaged 22.56 in cases with only cytoplasmic expression of YB-1 but averaged 28.20 in cases with cytoplasmic and nuclear expression of YB-1; P = 0.0477). In human osteosarcoma, nuclear localization of YB-1 protein was associated with the expression of P-gp, suggesting that YB-1 could be a prognostic marker for multidrug resistance in osteosarcoma.