Background: The precise mechanisms controlling homing of T effector (Teff) cells to the inflamed gut in Crohn's disease (CD) are still unclear, and clinical outcome data from patients with inflammatory bowel disease treated with the anti-alpha 4 beta 7 integrin antibody vedolizumab suggest differences between ulcerative colitis and CD. Methods: Expression of homing molecules was studied with flow cytometry and immunohistochemistry. Their functional role was investigated in in vitro adhesion assays and in a humanized mouse model of T cell homing to the inflamed gut in vivo. Results: Despite in vitro blockade of CD Teff adhesion to mucosal vascular addressin cell adhesion molecule-1 (MadCAM-1) and in contrast to previous observations in ulcerative colitis, anti-alpha 4 beta 7 treatment did not result in reduced Teff cell homing to the colon in vivo. However, the integrin alpha 4 beta 7 was expressed in higher levels on Teffs from patients with CD compared with controls, while its expression in the peripheral blood declined, and its expression in the intestine increased during the course of clinical vedolizumab treatment. Consistently, adhesion of CD Teffs to vascular cell adhesion molecule-1 (VCAM-1) was blocked by inhibition of alpha 4 and alpha 4 beta 7 in vitro. Moreover, in vivo homing of CD Teffs to the ileum was reduced by inhibition of alpha 4 and alpha 4 beta 7 integrins, but not alpha 4 beta 7 integrins. Conclusions: Our findings suggest that Teff cell homing to the ileum through the axis alpha 4 beta 7-VCAM-1 is an essential and nonredundant pathway in CD in vivo, possibly affecting efficacy of clinical treatment with antiadhesion compounds.