IgG anti-endothelial cell antibodies (AECA) were detected in 48.5% of patients with active systemic lupus erythematosus (SLE) and in 7% of patients during remission and were associated with the development of diffuse proliferative lupus nephritis. Sixteen AECA-positive patients were prospectively studied for 25.2 +/- 2.9 months. Serial AECA levels correlated with disease activity in 10 (62.5%) patients. Seven (43.8%) of 16 patients remained AECA positive during clinical remission. Among four episodes of disease exacerbation and 16 instances of clinical improvement, 85% (17 episodes) were accompanied by corresponding changes in the level of AECA, while corresponding changes in C3, anti-nuclear antibodies, and anti-double-stranded DNA antibodies were noted in 60, 60, and 80% of cases, respectively (p = not significant). AECA served as the only serologic marker of altered disease activity in five episodes, when C3, ANA, and anti-double-stranded DNA levels remained unaltered. We conclude that the level of AECA can serve as a marker of disease activity in SLE and that serial monitoring of AECA can complement other serologic parameters in the management of patients.