Poorly functioning dialysis access, whether graft or AV fistula, is a contributor to decreased efficiency of dialysis. Thrombosis results in the need for surgery and increases patient suffering. Objectives: To determine whether an advanced glycation end product (AGE), pentosidine, might serve as a marker of disease activity and thrombosis of dialysis access. These results were obtained during the blinded phase of a study to evaluate the efficacy of the antioxidant vitamin E in preventing vascular access thrombotic events. Methods: Twenty-six patients had blood drawn and access flow evaluated initially and at intervals for up to 1.5 years. In the study design, participants received either 800 IU vitamin E daily by mouth or identical placebo capsules. Blood plasma was examined to determine pentosidine content at baseline and every 3 m. Patients with an access flow of <800 mL/min were referred to a surgeon for evaluation. Access flow was evaluated by using the transonic flow meter and the saline release method. When sequential double dialyzers were used, saline injection method was applied post dialyzers, but before the venous drip chamber. Results: Mean initial level of pentosidine before the administration of vitamin E was 20.22 pmol/mg (range 5.04–59.62 pmol/mg). During the study period, 5 patients had procedures related to thrombosis performed on their accesses. Blood drawn before the thrombotic event showed an increase above baseline of 6.87 ± 15.47 (range −5.9 to +33.7 pmol/mg). Patients without events showed a mean decrease of 9.7 ± 12.24 pmol/mg pentosidine (p < 0.02). Conclusion: An increase in levels of pentosidine was associated with clotting of hemodialysis access. This change may reflect the effects of oxidative stress and/or thrombus formation. As these data are gathered from an ongoing double-blinded study, the effect of vitamin E on thrombus formation and pentosidine levels has not yet been ascertained.