During vascular disease and following injury, vascular smooth muscle cells (VSMC) proliferate and produce inflammation-promoting cytokines and chemokines. Similar phenotypic changes can be elicited in vitro by activation of Toll-like receptors (TLR) within VSMC. TLR-activated VSMC also produce IL-1α, but it is unknown whether endogenous IL-1α stimulates VSMC in an autocrine manner. Here we tested the hypothesis that endogenous IL-1α contributes to TLR-induced proliferation and chemokine release in human VSMC by using RNA interference to knock down IL-1α expression. Knockdown of IL-1α abolished TLR-induced proliferation and suppressed TLR4-induced release of monocyte chemoattractant protein-1 (MCP-1) by VSMC, indicating that endogenous IL-1α plays a crucial role in both responses. Serum, PDGF, FGF-2, and EGF each increased cellular IL-1α concentrations, and IL-1α knockdown inhibited serum- and PDGF-induced DNA synthesis, further indicating that endogenous IL-1α also contributed to VSMC responses to growth factors. IL-1 receptor antagonist, a competitive inhibitor of IL-1 receptor I (IL-1RI), also attenuated TLR-induced proliferation and both basal and TLR-induced MCP-1 expression, indicating at least a partial role of the IL-1RI in mediating these responses. The results support the hypothesis that autocrine actions of endogenous IL-1α, mediated at least in part via IL-1RI signaling, contribute to a proproliferative and proinflammatory phenotypic shift in TLR-activated human VSMC, which might play a pathogenic role in vascular disorders.