The prevalence of three different types of antiphospholipid antibody in 88 consecutive patients with systemic lupus were 27.2% for lupus anticoagulant (LAC), 31.8% for anticardiolipin antibody (aCL), and 13.6% for falsely positive serologic tests for syphilis (FPSTS). The three tests were correlated, thus confirming the overlapping specificities of this family of antibodies. Although FPSTS was not associated with any particular manifestation of systemic lupus, aCL correlated with thrombosis (p = 0.0001), thrombopenia (p = 0.009), neuropsychiatric features (p = 0.02) and membranous nephropathy (p = 0.001), while LAC correlated with thrombosis (p = 0.001) and hemolytic anemia (p = 0.04). The previously unreported association between membranous nephropathy and aCL might explain some features of the former, particularly the higher incidence of thromboembolic complications and the poorly known relation with renal vein thrombosis.