In addition to its calciotropic function, the secosteroid 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) has potent nonclassical effects. In particular, local production of 1,25D3 catalyzed by the enzyme 1α-hydroxylase (1α-OHase) may act as an autocrine/paracrine immunomodulatory mechanism. To investigate the significance of this in vascular tissue the expression and function of 1α-OHase in human endothelial cells was characterized. Immunohistochemical and in situ hybridization analyses show, for the first time, the presence of 1α-OHase mRNA and protein in endothelial cells from human renal arteries as well as postcapillary venules from lymphoid tissue. Reverse transcription–PCR and Western blot analyses confirmed the presence of 1α-OHase in primary cultures of human umbilical vein endothelial cells (HUVEC). Enzyme activity in HUVEC (318 ± 56 fmoles 1,25(OH)2D3/hr/mg protein) increased after treatment with tumor necrosis factor–α (1054 ± 166, P < 0.01), lipopolysaccharide (1381 ± 88, P < 0.01), or forskolin (554 ± 56, P < 0.05). Functional studies showed that exogenously added 1,25(OH)2D3 or its precursor, 25-hydroxyvitamin D3 (25(OH)D3), significantly decreased HUVEC proliferation after 72 h of treatment (33% and 11%, respectively). In addition, after 24 h treatment, both 1,25(OH)2D3 and 25(OH)D3 increased the adhesion of monocytic U937 cells to HUVEC (159% and 153%, respectively). These data indicate that human endothelia are able to produce active vitamin D. The rapid induction of endothelial 1α-OHase activity by inflammatory cytokines suggests a novel autocrine/paracrine role for the enzyme, possibly as a modulator of endothelial cell adhesion.