Data are few and conflicting about the prevalence and risk factors for antiphospholipid (aPL) antibodies in end-stage renal disease (ESRD). We studied the prevalence, risk factors and clinical manifestations of lupus anticoagulant (LA) and anticardiolipin antibodies (aCL) among ESRD patients (chronic hemodialysis (HD) patients and kidney transplant recipients) and blood donors. LA was assessed in a large cohort (n=180) of patients by the activated partial thromboplastin time (aPTT), dilute Russel's viper venom test (dRVVT) and lupus anticoagulant-sensitive aPTT reagent (PTT-LA). IgM- and IgG-aCL were measured by a solid-phase enzyme-linked immunosorbent assay (ELISA) in 111 patients (61.5%). The prevalence of aPL was low but, it was higher in ESRD than blood donors (8.8% (16/180) vs. 0%, P=0.005); the frequency of aCL was also higher in ESRD than controls (10.8% (12/111) vs. 0%, P=0.002). LA was similar in the study and control groups (2.2% (4/180) vs. 0%, NS). Among HD patients and kidney allograft recipients there was no difference in LA (3.9% (4/101) vs. 0% (0/79), NS) and aCL frequency (18.6% (8/43) vs. 5.9% (4/68), NS). aPL was not associated with sex, age, time on HD, post-transplantation follow-up, ESRD etiology, thrombotic or hemorrhagic events, or type of HD membrane; however, these findings must be interpreted with caution, given the low frequency of aPL. In one HD patient LA activity was associated with multiple thrombosis of the access graft and native veins. In summary, the prevalence of aPL in ESRD is low but nevertheless higher than controls; LA does not appear to be related to membrane bio-incompatibility and it may be linked to vascular thrombosis; the lack of concordance between LA and aCL was apparent. Further studies are needed to clarify the issue of aPL in ESRD. LA testing should be incorporated into the diagnostic evaluation of recurrent thrombotic episodes in patients on HD.