BACKGROUND. Idiopathic thrombocytopenic purpura (ITP) is an autoimmune disorder of blood haemostasis with thrombocytopenia. The incidence of bleeding in ITP patients is lower than may be predicted on the basis of the platelet count. The proportion of activated platelets in patients with untreated ITP and in splenectomized ITP patients in remission was evaluated. METHODS. Two groups of ITP patients were chosen. The first included untreated ITP patients and the second consisted of splenectomized ITP patients in remission. The proportion of activated platelets was determined for each group and compared with the control group. RESULTS. The untreated group had a significantly higher (p = 0.0042) proportion of activated platelets than the control group. The group of splenectomized patients in remission showed no significant difference in the proportion of activated platelets compared to the control group. CONCLUSIONS. ITP patients in remission and without therapy have a higher proportion of activated platelets than the healthy population, which could explain their low bleeding tendency. On the other hand, splenectomized ITP patients in remission have the same proportion of activated platelets in their blood as the healthy population and this emphasizes the primary importance of the spleen in ITP patients.