Platelet aggregation (PA) and erythrocyte deformability (ED) were studied in twenty patients undergoing TPN for major abdominal surgery. PA was studied by the method of Born. ED was evaluated using standard filters (Uni Pore) and recording the filterability time (FT). All the patients had normal basal values of both parameters. In those receiving TPN slight increases of PA and FT were noticed after the first post-operative day (ADP 1 muM = 31.29 + 6.4%, EPI 5 muM = 24.08 + 8.2%, COLL 0.5 mug/ml = 27.94 + 6.5%, FT = 35.5 + 3.5''). This trend was more pronounced on post-operative day 4, and was statistically significant on days 7, 14, and 19 (ADP = 49.76 + 8.8, EPI = 57.2 + 11%) (p 0.05), (COLL = 73.2 + 8.6%) (P 0.01), (FT = 65.5 + 10.5'') (p 0.05). Some days after stopping TPN, these values slowly returned towards normal. These changes were not observed in a control group of ten patients, not undergoing TPN. From our data the following conclusions can be drawn: 1) PA is progressively enhanced in surgical patients undergoing TPN and slowly returns to normal several days after oral feeding resumption. 2) There is a significant linear relationship between PA and FT in the post-operative phase.