Recent studies suggest that patients undergoing warm heart surgical procedures have reduced postoperative bleeding. To determine if this is due to differences in platelet activation, we measured platelet membrane glycoproteins (GPIb, GPIIb/IIIa, GMP 140), platelet fragments, and platelet counts before, during, and after normothermic (37 degrees C) or hypothermic (28 degrees to 30 degrees C) cardiopulmonary bypass. Cardiopulmonary bypass was associated with a significant decrease in platelet count, platelet membrane GPIb, and platelet fragments, and an increase in GMP 140 (p < 0.05). Normothermic cardiopulmonary bypass induced an early significant increase in granulocytes, whereas this was delayed until after rewarming in the hypothermic group. Mean 24-hour postoperative blood loss was 786 +/- 226 mL in the cold group versus 547 +/- 56 mL in the warm group (p = not significant). We conclude that cardiopulmonary bypass affects platelet activation and integrity and that these changes are similar in direction and magnitude for hypothermic and normothermic techniques.