OBJECTIVE Some phosphodiesterase (PDE)-inhibitors are believed to alter platelet count and function due to changes in intracellular cAMP. Whether newly developed (specific) PDE-inhibitors negatively influence platelet function in cardiac surgery should be investigated in a randomized study. METHODS Eighty patients undergoing aorto-coronary bypass grafting were divided into 4 groups and received either the new PDE-III-inhibitor piroximone (group 1), the PDE-III-inhibitor enoximone (group 2), epinephrine (group 3) or no inotropic support (control). PDE-III-inhibitors were given as a bolus followed by infusion until starting of cardiopulmonary bypass (CPB). In addition to platelet count and a thrombelastogram, platelet function was assessed by aggregometry (ADP, epinephrine, collagen). Measurements were done before, during and after CPB until the 1st postoperative day. RESULTS Platelet count and postoperative blood loss did not differ between the groups within the entire investigation period. Maximum aggregation and maximum gradient of platelet aggregation to all stimuli were not changed by either PDE-inhibitor enoximone or piroximone. CPB resulted in a significant decrease of all aggregation variables which was without differences due to treatment. Platelet aggregation recovered in the post-bypass period and exceeded baseline values on the 1st postoperative day. CONCLUSIONS It is concluded that enoximone and the new PDE-III-inhibitor piroximone do not affect platelet function and can be used before CPB without risking platelet-related bleeding in cardiosurgical patients in the perioperative period.