We report a case of a 2-mo-old girl with malignant osteopetrosis. Conventional radiological investigations of the skull and left hand showed the characteristic pattern of generalized sclerosis. Bone marrow immunoscintigraphy with 99mTc-labeled antibodies against nonspecific cross-reactive antigen (NCA) 95 was performed before and after bone marrow transplantation. Before transplantation, whole-body images showed bone marrow stores exclusively in the base of the skull. The rest of the skeleton did not reveal any hematopoietic activity. The liver and spleen showed increased antibody uptake as expected in extramedullary hematopoiesis. Repeat scintigraphy after bone marrow transplantation from her haploidentical father demonstrated an almost completely normalized tracer distribution corresponding to her clinical and hematological improvement. Bone marrow immunoscintigraphy appears to be an ideal complement to radiograph diagnostics in malignant osteopetrosis. In primary diagnosis, scintigraphy demonstrates the quantitative extent of bone marrow displacement. It also proves an ideal tool in monitoring the effectiveness of therapy after bone marrow transplantation.