IgG antiendothelial antibodies (IgG AEA), as measured by enzyme-linked immunosorbent assay, could be detected in serum samples of 38 out of 41 patients with systemic lupus erythematosus. Incubation of endothelial cells (EC) with interleukin-1 alpha (IL-1 alpha), in contrast to incubation with interferon gamma or tumor necrosis factor alpha, resulted in an enhanced IgG AEA binding. Immunoblotting revealed reactivity of AEA against a variety of EC antigens. The upregulation of IgG-AEA-binding reactivity to IL-1 alpha-stimulated EC was due to binding to antigens that were already expressed by unstimulated EC. The IgG-binding reactivity to both IL-1 alpha-stimulated and unstimulated EC was significantly higher in the serum of patients with joint or skin abnormalities as compared with patients without these manifestations. These data suggest that upregulated binding of IgG to EC induced by IL-1 alpha may play a role in immune vascular damage.