IgG antibodies reactive with human umbilical vein endothelial cells were found in 19 out of 28 patients with rheumatoid vasculitis (RV), in four out of 24 patients with rheumatoid arthritis (RA), in seven out of 10 patients with systemic lupus erythematosus (SLE), but not in healthy donors. In four patients with RV who were followed longitudinally, regression of vasculitic episodes coincided with decreasing titres of anti-endothelial antibodies (AEA). Binding activity to endothelial cells was observed in intact IgG and F(ab')2 fragments of IgG. AEA activity was unrelated to antibodies against nuclear, blood group or major histocompatibility complex antigens and did not involve immune complexes. AEA activity was not specific for endothelial cells since the AEA-positive sera and the IgG fractions prepared from these sera also reacted with fibroblasts. Adsorption of positive sera and corresponding IgG fractions with endothelial cells decreased the IgG binding reactivity on both fibroblasts and endothelial cells. These findings show that RV patients have IgG-AEA, and suggest that these antibodies may play a role in the pathogenesis of the disease.