We studied 50 chronic dialysis patients with end-stage renal disease. Mean platelet count was within normal limits. An inverse linear correlation was observed between pre-dialysis platelet count and serum creatinine (r=0.304, p=0.038). Dialysis caused a decrease in platelet count (216+/-80x10(9)/L, pre; 198+/-68, post; p=0.0001), and the higher the pre-dialysis platelet count, the greater the decrease (r=0.623, p=0.0001). Post-dialysis triglyceride decreased (1.67+/-1.27 mmol/L, pre; 1.23+/-0.96, post; p=0.0001). Tissue factor pathway inhibitor (TFPI) antigen plasma level was higher in uremic patients than in controls (114+/-42 ng/ml vs. 64+/-12, p=0.0001). TFPI increased 2.3 times following dialysis and such an increase was directly correlated with post-dialysis plasma heparin concentration (r=0.571, p=0.0002) and inversely correlated with post-dialysis triglyceride variation (r=0.407, p=0.005). Six of fifty patients (12%) had anti-heparin/platelet factor 4 antibodies (Hab), 3 IgG, and 3 IgM. Female sex and the use of cuprophane membranes were more frequent among Hab-positive patients (p=0.0001), while a lower percentage of them were on anti-aggregating drugs as compared to Hab-negative patients (p=0.002). Only one Hab-positive patient was slightly thrombocytopenic and none showed bleeding or thrombotic manifestations. Serum albumin and y globulin decreased following dialysis in Hab-positive patients, while the opposite was seen in those Hab-negative (-2.47+/-1.72 g/L, vs. 0.21+/-1.77, p=0.001 and -0.48+/-0.60 g/L vs. 0.64+/-0.97, p=0.007, respectively). In vivo factors other than Hab are involved in the development of heparin-induced thrombocytopenia. Besides a blunted immunological response, increased levels of TFPI, the use of anti-aggregating drugs, and the observed behavior of serum proteins might play a role in this regard.