The hypothesis is: Allogeneic hematopoietic cell transplantation (HCT) can achieve sustained remissions in patients with refractory CD, and can be done safely. The specific aims are: 1) To evaluate the safety and efficacy of allogeneic HCT as treatment for refractory CD. 2) To evaluate treatment effect on CD activity/severity using the Crohn's Disease Activity Index (CDAI) and the Simple Endoscopic Score for CD (SES-CD). 3) To evaluate safety by scoring regimen-related toxicities, time to engraftment, infectious complications, acute and chronic Graft-versus-Host Disease (GVHD), and treatment-related mortality. 4) To evaluate the effect on quality of life.