Bone marrow and peripheral blood samples from 60 patients with suspected bone sarcoma were examined for the presence and number of micrometastatic osteosarcoma cells by a sensitive immunomagnetic detection assay, using in parallel two osteosarcoma-associated antibodies. Forty-nine of the patients had osteosarcoma, and of these, as many as 31 (63%) had tumor cells in bone marrow, in many cases with a high number of cells. Only four (8%) were positive also in blood. None of 38 control bone marrow samples were positive, including 11 from patients with suspected bone sarcoma at time of sampling who later were found not to have osteosarcoma. Fifteen of 28 patients without overt metastases at primary diagnosis (54%) were positive, 12 of whom had localized high-grade primary tumors in the extremity. Four of these have relapsed compared with none of 10 negative patients. In the group of 22 patients with extremity localized nonmetastatic osteosarcoma, information was available on the histologic response to preoperative chemotherapy in 15 patients. None of the three patients in the bone marrow-negative group who had a poor response to chemotherapy have relapsed, whereas two of the four poor responders in the bone marrow-positive cohort are dead of disease. Among 12 patients with overt metastasis at primary diagnosis, 11 (92%) were positive in bone marrow with a very high number of osteosarcoma cells. The immunomagnetically isolated cells were further characterized by the use of fluorescent latex microparticles with surface-bound antibodies targeting different membrane markers. Moreover, in cases with numerous osteosarcoma cells in bone marrow attempts to grow the selected cells in vitro were successful in two of eight attempts, and in two of five cases, isolated cells produced tumors with osteosarcoma characteristics in nude mice. In conclusion, already at primary diagnosis, a very high fraction of osteosarcoma patients had malignant cells in bone marrow, and a correlation between the presence of tumor cells, clinical stage, and disease progression was found. The data show the clinical potential of this immunomagnetic method. Attempts to subgroup osteosarcoma patients for more individualized treatment based on the presence of micrometastatic cells should be studied in a larger cohort of patients.