Osteosarcoma, one of the most frequent secondary malignancies after the treatment of young patients with cancer, has only very rarely been observed in association with hematopoietic stem cell transplantation (HSCT). We report four patients who were identified by searching the database of the Cooperative Osteosarcoma Study Group (COSS) for patients whose osteosarcoma arose following HSCT. Transplant indications had been acute lymphoblastic leukemia (3). and sickle cell disease (1). and the stem cell source was bone marrow in all cases (three allogeneic, one syngeneic). All four had received chemotherapy with alkylators as part of their conditioning regimen and/or first line therapy. The conditioning regimen included total body irradiation in three patients. The osteosarcomas arose at the age (adolescence) and sites (around the knee) typical for the disease. All four patients received chemotherapy as part of multimodal osteosarcoma treatment, and all four are currently alive, three in continuous remission at 5 7/12, 2 11/12, and 0 6/12 years and one with relapsed osteosarcoma at 4 1/12 years. One of the osteosarcoma-free survivors suffered a third malignancy, myelodysplastic syndrome. Osteosarcoma should be included among the secondary malignancies that can arise following HSCT. Multi-modal therapy according to guidelines for de novo osteosarcoma can lead to long-term survival in selected patients.