The aim of the present study was to evaluate the efficiency and side effects of mifamurtide in childhood osteosarcoma (OS). In total, 477 doses of 2 mg/m(2) intravenous (IV) mifamurtide, along with paracetamol as a premedication, were given to 15 patients with primary nonmetastatic OS after complete surgical resection and to 3 patients with progressive OS. The most common side effects encountered in the patients were chills and fever (17/18). These reactions were observed in 4 patients during the administration of each dose, in a single patient during the last administration, and in the remaining 12 patients during the first or initial 2 administrations. Headache, myalgia, and arthralgia were observed in 2 patients during each infusion. Headache was observed in 1 patient with additional hearing loss during the first 2 infusions. One patient had back pain occuring within the first infusion. Of the 15 patients with primary nonmetastatic OS and treated with the addition of mifamurtide to chemotherapy, 13 showed a complete remission, and 2 patients were still under treatment with a complete remission. Of 3 patients with progressive disease. 2 died while the disease progressed further in the third case over a 51-month period. The 3-year overall survival and event-free survival distributions were 87.5% (mean follow-up time, 46.12: 95% confidence interval, 37.79-52.45 mo) and 75.6% (mean follow-up time. 31.30: 95% confidence interval. 26.54-36.06 mo), respectively. We consider that mifamurtide therapy is a safe and welltolerated agent in childhood OS.