We retrospectively investigated the association between platelet autoantibody specificity and response to intravenous immunoglobulin G (IVIG) in 17 patients with immune thrombocytopenia (ITP). Platelet-associated antibodies against glycoprotein (GP) IIb/IIIa, GPIb/IX, and GPIa/IIa were detected in 13, 10, and 8 patients, respectively. A response occurred in 7 of 7 patients without anti-GPIb/IX, but in only 3 of 10 patients with anti-GPIb/IX (p< 0.01). There was no difference in the response rates in patients with or without anti-GPIIb/IIIa or anti-GPIa/IIa. We conclude that ITP patients with anti-GPIb/IX may be less responsive to IVIG.