To investigate the relationship between dialysis access thrombosis (arteriovenous grafts [AVG] and arteriovenous fistulas [AVF]) and the presence of elevated concentration of immunoglobulin G-anticardiolipin antibody (IgG-ACA), we conducted a cross-sectional study of all patients, in a single dialysis facility, who had a minimum of 6 months of uninterrupted hemodialysis. Episodes of thrombosis of AVGs and AVFs in the preceding 30 months were documented and each patient's IgG-ACA titer was determined. Sixteen of 74 patients with AVGs (22%) had a raised titer of IgG-ACA compared with only one of 17 patients (6%) with AVFs. In the patients with AVGs the odds ratio for patients with raised IgG-ACA titer to have experienced two or more episodes of thrombosis, compared with none or only one episode, was 3.7 (95% confidence interval, 1.2 to 11.8; P < 0.04). No events of AVF thrombosis were encountered during the same period. We conclude that in end-stage renal disease patients undergoing hemodialysis, there is a greater prevalence of elevated IgG-ACA titer in patients with AVGs than in patients with AVFs, and this in turn is associated with increased odds of having had recurrent AVG thrombosis.