Platelet glycoprotein (GP) IIb/IIIa has an important role in platelet aggregation. A polymorphism of platelet GPIIIa (PlA2, also called HPA1b) has been associated with a higher risk of thrombosis, but its implication in heparin-induced thrombocytopenia (HIT) is unclear. To investigate the hypothesis that the PlA2 polymorphism influences the prothrombotic effects of HIT, we conducted a prospective study of 66 consecutive patients with a laboratory diagnosis of HIT. The end point of the study was the diagnosis of a thrombus within 30 days of the positive HIT test result. The Diagnostica Stago (Asnières, France) enzyme-linked immunosorbent assay was used to detect HIT antibodies, and a polymerase chain reaction assay was used to detect the PlA2 polymorphism. Of the 66 patients, thrombotic complications developed in 27 (41%). Patients with the PlA2 allele demonstrated a significantly higher thrombosis risk than did patients without (69% vs 32%; P = .0088; odds ratio, 4.68; 95% confidence interval, 1.39-15.72). The risk was stronger for arterial thrombosis and for patients 60 years or older. There was a significant association between the PlA2 polymorphism of GPIIIa and the risk of thrombosis in patients with HIT antibodies.