Objective: To study serum levels of von Willebrand factor antigen (vWF:Ag) and anti-endothelial cell antibodies (AECA) in pregnancies with systemic lupus erythematosus (SLE) compared to healthy pregnancies and to evaluate their correlation with disease activity and fetal outcome. Methods: Twenty-four patients with SLE were compared with 26 healthy pregnancies. vWF:Ag and AECA levels were measured by ELISA. Results: In lupus pregnancies vWF:Ag and AECA levels were significantly elevated compared to healthy pregnancies (vWF:Ag: 255 ± 160%, 10/24 positive vs 135 ± 590/o, 1/26 positive, p = 0.006; AECA Binding Index: 36 ± 29%, 5/24 positive vs 17 ± 16%o, 1/26 positive, p = 0.002). When samples taken during follow-up were evaluated, vWF:Ag was positive in 21/27 (78%) of the samples during exacerbations compared to 4/25 (16%) in remission. Only 5/27 (19%) of the samples during exacerbations and none during remission were positive for AECA. Samples during three pregnancy failures were positive for vWF:Ag, but not for AECA. Of the 13 samples taken from the mothers of 4 babies with neonatal lupus, 10 were positive for vWF:Ag and three for AECA. No correlations of anti-cardiolipin antibodies (aCL) were seen either with vWF:Ag or AECA titres. Conclusion: Compared to healthy pregnancies, vWF:Ag and AECA levels are increased in lupus pregnancies. vWF: Ag levels also correlated with active disease.