In flares of systemic lupus erythematosus (SLE), endothelial cells activated by immune stimuli are potential participants in the inflammatory processes, which contribute to injury. Elevated levels of circulating endothelial cells (CEC) may be a proxy for vascular injury, as demonstrated in patients with sickle cell anemia during acute crises. In active SLE, CEC levels in peripheral blood are elevated (vs healthy controls and correlate with plasma C3a). CEC may reflect widespread unrecognized, ongoing injury despite the absence of clinical stigmata of vasculitis in patients with SLE.