Platelets from 42 patients (platelet counts median 42 × 109/L, range 3–223 × 109/L) with chronic idiopathic autoimmune thrombocytopenia (cAITP) were investigated for P-Selectin expression and PAC-1 binding. The results showed that the levels of P-Selectin positive platelets (n = 20) were higher in cAITP than in controls (P < 0.0001), and correlated with platelet counts (P = 0.04). PAC-1 binding was increased in only six patients, and not correlated with platelet counts. There was no correlation of P-Selectin or PAC-1 with detectable platelet antibodies. Thus, platelets are activated in cAITP, but platelets, characterized by PAC-1 binding, are rare. These are either needed to maintain vascular integrity, or underwent premature sequestration. Pediatr Blood Cancer 2006;47:694–696. © 2006 Wiley-Liss, Inc.