OBJECTIVE This review updates the differential diagnosis between inherited and acquired immune thrombocytopenic purpura as well as clinical practice on the initial diagnosis of children with the disease. RESULTS A diagnosis of immune thrombocytopenic purpura may be based on an evaluation of the history, physical findings such as petechiae, bruising and mucous membrane bleeding, examination of peripheral blood films stained with Wright's or May-Grünwald-Giemsa, determination of blood counts, platelet size and appearance. Recently, diagnostic assays have been developed to detect platelet-bound antibodies. The sensitivity of these assays, however, is suboptimal, with a positive predictive value of 80-83%. If the diagnosis of immune thrombocytopenic purpura is in question due to the presence of atypical features, or if a patient with findings typical of the disease does not respond to therapy, bone marrow aspiration and biopsy are indicated to confirm the diagnosis. CONCLUSIONS The diagnosis of immune thrombocytopenic purpura is a process of elimination of other sources of thrombocytopenia. If the criteria discussed above are inconclusive and if the patient does not respond to therapy in 6-12 months (this is especially true in children) then a bone marrow aspiration is required to confirm the diagnosis, especially before initiating corticosteroid therapy.