Purpose: The complexity of sarcoma has led to the need for patient selection via in vivo biomarkers. Tumor endothelial marker-1 (TEM-1) is a cell surface marker expressed by the tumor microenvironment. Currently MORAb-004 (Ontuxizumab), an anti-TEM-1 humanized monoclonal antibody, is in sarcoma clinical trials. Development of positron emission tomography (PET) for in vivo TEM-1 expression may allow for stratification of patients, potentially enhancing clinical outcomes seen with Ontuxizumab. Results: Characterization of cell lines revealed clear differences in TEM-1 expression. One high expressing (RD-ES) and one low expressing (LUPI) cell line were xenografted, and mice were injected with 89Zr-Ontuxizumab. PET imaging post-injection revealed that TEM-1 was highly expressed and readily detectable in vivo only in RD-ES. In vivo biodistribution studies confirmed high radiopharmaceutical uptake in tumor relative to normal organs. Experimental Design: Sarcoma cell lines were characterized for TEM-1 expression. Ontuxizumab was labeled with 89Zr and evaluated for immunoreactivity preservation. 89Zr-Ontuxizumab was injected into mice with high or null expressing TEM-1 xenografts. In vivo PET imaging experiments were performed. Conclusion: 89Zr-Ontuxizumab can be used in vivo to determine high versus low TEM-1 expression. Reliable PET imaging of TEM-1 in sarcoma patients may allow for identification of patients that will attain the greatest benefit from anti-TEM-1 therapy.