• Altered immune function has previously been reported in children with Idiopathic thrombocytopenic purpura (ITP). Some prior studies have shown that acute and chronic ITP might be differentiated by certain immunologic tests. We investigated lymphocyte subsets in 12 children with ITP (six acute and six chronic) using murine monoclonal antibodies. Although the mean percentages of B lymphocytes and total number of T lymphocytes, suppressor T cells, and helper T cells were similar in patients with acute and chronic ITP and in controls, the values for many children with ITP fell outside of the normal range. The distribution of values was similar among the patients with acute and chronic ITP. These results suggest a heterogenous nature of immune-mediated thrombocytopenia in childhood.