Mifamurtide is an immune macrophage stimulant that when added to standard chemotherapy has demonstrated survival benefit for newly diagnosed osteosarcoma. The objectives of this study were to investigate the cost-effectiveness of adding mifamurtide to standard three- or four-agent chemotherapy for high-grade, resectable, nonmetastatic osteosarcoma following surgical resection and the issues of obtaining robust cost-effectiveness estimates for ultra-orphan drugs, given the shortage of data.