The purpose of this study is to establish a registry of all children with severe, malignant osteopetrosis who are treated with Actimmune (IFN-g 1b or Interferon gamma-1b) to monitor the effects of IFN-g 1b on preventing progression of this disease and to follow the safety of patients receiving it on a long-term basis. In addition, evaluation of the possible effect of Actimmune therapy on the humoral response to normal childhood vaccinations in this same patient population will be examined.Interferon gamma is a substance that the body makes naturally.