AECA were detected in 25 of 71 patients with type 1 diabetes mellitus and in two of 33 healthy subjects. Patients with diabetes of < 1 year duration and those with long-standing disease had the highest levels of these antibodies. Inhibition studies suggest that at least part of the AECA reactivity is due to cross-reactive anti-ssDNA antibodies. AECA-positive sera were able to increase intercellular adhesion molecule-1 (ICAM-1) and E-selectin on human umbilical vein endothelial cells (HUVEC). Increased binding of polymorphonuclear (PMN) cells was also found to accompany raised E-selectin expression. Soluble ICAM-1 and E-selectin were also found to be increased in the sera of AECA-positive patients. An effect of AECA on endothelial cell function is suggested in diabetes mellitus.