This is a Phase 2 protocol evaluating the safety and efficacy of standard dose insulin-like growth factor-1 (IGF-1) for the treatment of growth failure in patients diagnosed with X-linked severe combined immunodeficiency (XSCID). This condition is a fatal inherited immunodeficiency caused by defects in the common cytokine receptor gamma chain (gc), a subunit shared by many cytokine receptors. The common gamma subunit signals through the Janus kinase/signal transducers and activators of transcription (JAK/STAT) pathway, a complex intracellular signaling pathway used by many cytokines and growth factors, including growth hormone (GH). Studies have suggested that the gc defect may result in hyporesponsiveness to GH. This is supported by a report of GH hyporesponsiveness in an XSCID child that was successfully ameliorated following immune reconstitution using bone marrow transplantation (BMT). Haplo-identical BMTs for XSCID children often achieve only partial immune reconstitution, and many BMT recipients experience ongoing problems with growth failure, achieving heights well below 2 standard deviations for their chronological age. It is possible that in these partially corrected conditions, administration of IGF-1, a substance the body produces downstream in response to GH, may achieve an improved growth response.