Hemodialysis patients suffer from premature atherosclerosis and various thrombotic and thromboembolic phenomena. Despite enhanced in vivo platelet activity, it is unclear whether in vitro platelet aggregation is increased or reduced in this population. We studied platelet aggregation in response to adenosine diphosphate and to epinephrine in 20 chronic hemodialysis patients and 21 controls. In the patient group the in vitro platelet aggregation was significantly enhanced in response to both aggregating agents. Our study demonstrated that in vitro platelet aggregation is enhanced in patients with chronic renal failure undergoing hemodialysis.