To evaluate whether increased plasma homocysteine concentrations (hyperhomocysteinemia) are associated with thrombosis of arteriovenous (AV) grafts, we determined plasma homocysteine, plasma and erythrocyte folate, plasma vitamin B12, and vitamin B6 (pyridoxal-5'-phosphate [PLP]) in 48 patients (45 black patients and three white patients) with end-stage renal disease who received hemodialysis.