Low-grade osteosarcomas comprise a distinct subset of osteosarcomas. They may occasionally dedifferentiate into high-grade tumors, typically in the form of high-grade osteosarcoma, which are histologically indistinguishable from conventional osteosarcomas. MDM2 and CDK4 are often amplified in low-grade osteosarcomas and their dedifferentiated counterparts, and the encoded proteins are accordingly overexpressed. As MDM2/CDK4 expression was reportedly rare in conventional osteosarcoma, we hypothesized that these markers may help separate dedifferentiated osteosarcoma from the conventional type. To test this, we performed MDM2 and CDK4 immunohistochemistry on 81 primary and 26 recurrent/metastatic high-grade osteosarcomas and correlated these data with the histology of the primary resection material, with particular attention to the potential presence of any coexisting low-grade osteosarcomatous components. MDM2 and CDK4 coexpression was identified in 7 cases, and on careful histologic review 6 of them were discovered to contain foci of coexisting low-grade elements. One case was a known dedifferentiated parosteal osteosarcoma, and the remaining 5 cases were newly identified dedifferentiated osteosarcomas in which the limited low-grade components were originally unrecognized. An additional 11 cases expressed either marker alone, whereas the remaining 89 cases were negative for both markers; no resection material from these 100 cases presented with a low-grade component. MDM2/CDK4 gene amplification status, determined by quantitative polymerase chain reaction in selected cases, was largely concordant with immunoexpression. Our data suggest that MDM2 and CDK4 coexpression in high-grade osteosarcomas is sensitive and specific to those that progressed from low-grade osteosarcomas, and immunohistochemistry may help identify this dedifferentiated subgroup to facilitate accurate subclassification.