The interaction of proinflammatory type 1 T helper (Th1) cells expressing the CD40 ligand (CD154) with endothelial cells expressing the corresponding receptor (CD40) may play an important role in chronic inflammation including arteriosclerosis. Here we demonstrate that activation of CD40 in human cultured endothelial cells (e.g. by interaction with freshly isolated human T cells) not only up‐regulates expression of various adhesion molecules, chemokines and cytokines, but within 12–24 h also causes the release of bioactive interleukin‐12 (IL‐12 p70) through induction of IL‐12 p40 synthesis. IL‐12 p35, on the other hand, appears to be constitutively expressed in these cells. Despite enhancing expression of the other gene products, cytokines such as interferon‐γ (IFN‐γ) or tumor necrosis factor‐α, alone or in combination, failed to induce IL‐12 p40 expression, whereas IFN‐γ markedly augmented CD154‐induced IL‐12 p40 and p70 release. Of note was that the magnitude of CD154‐induced IL‐12 synthesis in the cultured endothelial cells was comparable to that evoked in freshly isolated human monocytes. This CD40‐mediated induction of endothelial IL‐12 synthesis may thus lead to an enhanced activation of the adherent CD154‐expressing Th1 cells, thereby fuelling the proinflammatory response.