The blockade of the platelet integrin glycoprotein (GP) IIb/IIIa has proved to be an effective antiplatelet therapy. Profound thrombocytopenia has repeatedly been described as an adverse effect in patients treated with GP IIb/IIIa inhibitors, but its mechanism has not been elucidated yet. With use of flow cytometry, the activation status of platelets was monitored in 26 patients presenting with acute myocardial infarction who were treated with the GP IIb/IIIa inhibitor abciximab alone or in combination with the fibrinolytic agent reteplase. Fibrinogen and PAC-1 (a GP IIb/IIIa activation-specific monoclonal antibody) binding, as well as P-selectin expression on unstimulated platelets were constant in 25 patients throughout a follow-up of 7 days. In 1 patient (D.F.), the percentage of platelet-binding fibrinogen increased from 2.2% to 17.8%, for PAC-1 from 2.8% to 13.2%, and for P-selectin expression from 10.2% to 58.3% 10 minutes after the start of treatment. Furthermore, D.F. had a decrease in single platelet count in ethylenediaminetetraacetic acid-, citrate-, and heparin-anticoagulated and native blood. Blood films revealed platelet aggregates. In vitro testing of D.F.'s blood 2 and 4 weeks after initial admission demonstrated a reinduction of fibrinogen and PAC-1 binding to platelets, an increase of P-selectin expression, and formation of platelet aggregates following exposition of platelets to abciximab in vitro. In summary, this report describes the induction of platelet activation by a GP IIb/IIIa inhibitor in vivo and reinduction in vitro in direct association with thrombocytopenia. Platelet activation by GP IIb/IIIa inhibitors may be one potential mechanism for GP IIb/IIIa inhibitor-induced thrombocytopenia.