Platelet consumption and platelet kinetics during hemodialysis were quantified in Yorkshire pigs with In-111 labeled platelets. Six anesthetized pigs (20–25 kg) were hemodialyzed at 150 ml/min for 3 hr. All pigs were injected with autologous In-111 labeled platelets (300–420 μCi) 24 hr before dialysis and were systemically heparinized (ACT > 400 sec) before cannulation. Hemodialysis was instituted with a Drake-Willock hemodialysis machine and a hollow fiber dialyzer (Cobe4, 0.6 m2). In vitro sham dialysis was carried out at 150 ml/min for 3 hr with six more dialyzers in a flow-loop with the blood reservoir maintained at 37°C. In vitro thrombogenicity over-estimates (10-fold) in vivo values. In both systems, platelet deposition on dialyzers reached a steady state, suggesting a constant rate of thrombus formation and embolization in the hollow fiber system. The relative thrombus distribution after 3 hr of dialysis was similar in both systems, with adherent thrombi in the entry and exit ports and highest numbers in the midsection of the hemodialyzer. Biodistribution after 3 hr of dialysis indicated that thrombosis of the hemodialyzer and arterial and venous traps as well as embolization reduced the platelet pool in the blood and increased platelet emboli in lung, brain, kidneys, and skeletal muscle, as measured by the In-111 labeled platelets.