This trial is investigating the possible therapeutic effect of transplanting donated bone marrow cells into patients with severe Crohn's Disease, an autoimmune inflammatory bowel disease. The study is being conducted by the Fred Hutchinson Cancer Research Centre at the University of Washington, in Seattle. All participants will have severe Crohn’s Disease that has not responded to other treatments. They will initially be treated with low dose radiation and chemotherapy (to suppress the immune system), then transplanted with bone marrow from a matched, healthy donors. The patient will continue to get immunosuppressive therapy (tacrolimus) for three months following the bone marrow transplant. Participants will be monitored regularly over the next five years. The trial aims to enrol 12 patients by April 2014.