Vascular access thrombosis (VAT) is frequent in some hemodialysis patients. Antiphospholipid antibodies (APL) have been involved in thrombosis, and have been reported to be present in a high proportion of patients with chronic renal failure. We studied the relationship between APL and thrombosis in 97 hemodialysis patients (HD). Lupus anticoagulant (LA) was assessed by activated partial thromboplastin time (APTT) and by tissue thromboplastin inhibition assay (TTI). IgG-anticardiolipin (ACA) was measured by a solid phase ELISA. The prevalence of APL was 31%; LA was found in 16.5% and was detected in all cases by TTI. Only one patient was positive for APTT. ACA was found in 15.5%. Only one patient was positive for LA and ACA. We found no relation between APL and age, length of time on dialysis, sex, type of dialysis membrane, drugs, and chronic B and C hepatitis. A high prevalence of APL was found in patients with undetermined nephropathy. When histories of thrombosis were examined, VAT was found to be significantly more frequent in patients with LA than in patients without LA (62% vs. 26%; P = 0.01). This relation was not present with ACA. Since VAT is one of the most frequent causes of morbidity for HD, diagnostic evaluation of VAT in HD should now include assay for LA.