Aims: We evaluated modifications in formation of heterotypic platelet-leukocyte aggregation induced by dialysis through cellulosic or synthetic membranes and evaluated the effects of such procedures promoting adhesive interactions between leukocytes and normal endothelial cells (ECs). Methods: Samples were obtained from arterial and venous lines at baseline, after 15 and 120 min of hemodialysis. Heterotypic aggregation was assessed using flow-cytometric techniques. Experiments to determine leukocyte adhesion to ECs were performed in parallel plate perfusion chambers at 450 s<sup>–1</sup>. Results: Patients dialyzed with a cellulosic membrane showed a significantly higher baseline granulocyte heterotypic aggregation (median 22.5%, range 8.6–32%) versus healthy subjects (median 10%, range 3.2–14.6%; p < 0.05). Granulocyte heterotypic aggregation values remained increased throughout the hemodialysis session not only in the arterial line (median 18 and 24.5%, range 7–30 and 8.7–36% at 15 and 2 h, respectively) but also in the venous line (median 20 and 25%, range 8.6–32 and 11.5–35% at 15 min and 2 h, respectively). Basal lymphocytes heterotypic aggregation values observed in uremic patients were 6% (0.1–7.1%) versus 1.0% (0.5–2.8%) in the control group (p < 0.05). The increase remained during the hemodialysis session both in the arterial line (median 5 and 4%, range 0.2–14 and 0.5–7.1 % at 15 min and 2 h, respectively) and in the venous line (median 7 and 7%, range 1.4–14 and 0.5–10.6% at 15 min and 2 h). In contrast, patients dialyzed with a synthetic membrane showed a decreased basal granulocyte heterotypic aggregation compared to healthy subjects (median 3.5 vs. 10%, range 2.8–7 vs. 3.2–14.6%, respectively; p < 0.05). For lymphocytes, basal heterotypic aggregation values were 0.2% (range 0.1–0.5%) in dialyzed patients vs. 0.98% (range 0.5–2.8%) in healthy subjects (p < 0.05), without changes throughout the dialysis session. Changes in leukocyte adhesion during hemodialysis did not reach statistical significance with either hemodialysis membrane. Our studies confirm a differential activation of platelets and leukocytes depending on the nature of the dialysis membranes. However, activation of circulating cellular elements by hemodialysis procedures did not enhance cross-talk interactions between leukocytes and unaltered ECs.