HLA-matched bone marrow transplantation is an effective form of treatment for some patients with malignant osteopetrosis, a defect of osteoclast function. Following transplant, normal osteoclasts differentiate from donor-derived marrow stem cells and can function normally in some of these patients. For patients without an HLA-matched marrow donor, pharmacologic treatments have not yet proved effective. This article demonstrates that normal osteoclast function can be obtained following the transplantation of HLA-nonidentical marrow that has been purged of T lymphocytes in vitro.