A previously healthy 34-year-old woman, was diagnosed as having systemic lupus erythematosus (SLE), with membranous glomerulopathy which improved rapidly. Neither lupus anticoagulant nor anticardiolipin antibodies were detected in her plasma. After three months of total remission, she developed a severe pulmonary thromboembolism for which no specific biological cause was found. Her plasma was analysed for different antiphospholipid antibodies: lupus anticoagulant and anticardiolipin antibodies were again negative. Using an ELISA prepared with either five different anionic phospholipids or zwitterionic phosphatidylethanolamine, solely an anti-phosphatidylethanolamine IgG was discovered in her plasma. In lupus patients, the presence of antiphospholipid antibodies is now well recognized as a high risk factor for repeated thrombosis and/or recurrent abortions. This case suggests that the presence of antiphosphatidylethanolamine antibody should be investigated in cases of unexplained thrombosis in SLE, where the usual clinical and biological investigations have failed to shed light.