Patients treated with the immunosuppressive drug tacrolimus (FK506), which binds FK506 binding protein 12 (FKBP12) and then inhibits the calcium-dependent phosphatase calcineurin, exhibit decreased regulatory T cells, endothelial dysfunction, and hypertension; however, the mechanisms and whether altered T-cell polarization play a role are unknown. Tacrolimus treatment of mice for 1 week dose-dependently decreased splenic CD4(+)/FoxP3(+) (regulatory T cells), increased splenic CD4(+)/IL-17(+) (T-helper 17) cells, and caused endothelial dysfunction and hypertension. To determine the mechanisms, we crossed floxed FKBP12 mice with Tie2-Cre mice to generate offspring lacking FKBP12 in endothelial and hematopoietic cells only (FKBP12EC knockout [KO]). Given the role of FKBP12 in inhibiting transforming growth factor-β receptor activation, Tie2-Cre-mediated deletion of FKBP12 increased transforming growth factor-β receptor activation and SMAD2/3 signaling. FKBP12EC KO mice exhibited increased vascular expression of genes and proteins related to endothelial cell activation and inflammation. Serum levels of the proinflammatory cytokines IL-2, IL-6, interferon-γ, IL-17a, IL-21, and IL-23 were increased significantly, suggesting a T-helper 17 cell-mediated inflammatory state. Flow cytometry studies confirmed this, because splenic levels of CD4(+)/IL-17(+) cells were increased significantly, whereas CD4(+)/FoxP3(+) cells were decreased in FKBP12EC KO mice. Furthermore, spleens from FKBP12EC KO mice showed increased signal transducer and activator of transcription 3 activation, involved in T-helper 17 cell induction, and decreased signal transducer and activator of transcription 5 activation, involved in regulatory T-cell induction. FKBP12EC KO mice also exhibited endothelial dysfunction and hypertension. These data suggest that tacrolimus, through its activation of transforming growth factor-β receptors in endothelial and hematopoietic cells, may cause endothelial dysfunction and hypertension by activating endothelial cells, reducing regulatory T cells, and increasing T-helper 17 cell polarization and inflammation.