Although amrinone produces thrombocytopenia, no information is available regarding the acute effects of milrinone on platelets. Therefore, we evaluated the effects of milrinone on platelet number and function in cardiac surgical patients. Twenty-seven patients were studied during cardiac surgery requiring cardiopulmonary bypass (CPB). Patients were randomized to receive no milrinone (n = 10), or milrinone (n = 17) at a loading dose of 50-75 micrograms/kg in the CPB circuit followed by 0.5-0.75 micrograms.kg-1.min-1 for 12-24 h. Bleeding times and blood samples for coagulation studies were obtained prior to induction, and at 2 and 24 h after CPB. In both groups, platelet counts decreased significantly from the baseline at 2 and 24 h after CPB, and bleeding time increased significantly from the baseline at 2 and 24 h after CPB. No significant thromboelastoplasty (TEG) changes were observed in either group, and there were no significant differences in platelet aggregation or chest tube drainage between the groups. Acute milrinone administration did not cause significant changes in platelet number or function in patients undergoing cardiac operations requiring CPB, beyond the usual adverse effects of cardiac surgery and CPB.