Allogeneic hematopoietic stem cell transplantation represents the only curative option for malignant infantile osteopetrosis (MIOP), a rare disease of infants and young children, characterized by excessive accumulation of mineralized bone and abnormal hematopoiesis. We report a case of successful engraftment and stable full-donor chimerism in a patient with MIOP who underwent unrelated donor cord blood transplantation (CBT). The donor was 2-loci human leukocyte antigen (HLA)-mismatch. After a conditioning regimen based on the combination of busulfan, cyclophosphamide, total body irradiation, and antithymocyte globulin, the patient received a dose of 3.85 × 107/kg of nucleated cells. Neutrophil and platelet engraftment had been achieved by day +33 and +82, respectively, and the patient was discharged home on day +89. A successful engraftment of donor hematopoiesis was demonstrated and the child experienced grade II acute graft-vs.-host disease (GVHD) involving the skin only. A remarkable but non-progressive decrease in lumbar spine bone mineral density was observed in the first nine months post-transplant. This case suggests that unrelated donor CBT may be a feasible option in case of unavailability of a fully HLA-matched related or unrelated donor.