Although changes in the number and function of regulatory T lymphocytes have been reported in primary immune thrombocytopenia (ITP), no study has investigated whether quantification of these cell types in peripheral blood could be used as early predictive marker of treatment outcome. And, it is not clear whether any change occurs in peripheral blood memory B lymphocyte levels in ITP. Hence, the aim of this study was to investigate the percentage of regulatory T lymphocytes and memory B lymphocytes in peripheral blood of ITP patients compared to controls, and also examine whether these levels have any significant predictive value for therapy outcome. A total of 20 newly diagnosed, untreated patients with ITP and 20 healthy controls were included. Flow cytometric analyses of lymphocyte subtypes in the peripheral blood were performed in specimens obtained from patients at the time of diagnosis and one month after the therapy initiation. First line corticosteroid (1 mg/kg/day methylprednisolone) therapy or splenectomy as second line treatment was performed, and patients were followed up for 3 years. Percentage of regulatory T lymphocytes (0.25 +/- 0.17% vs. 1.14 +/- 0.77%, P < 0.0001, n = 20) and percentage of memory B lymphocytes (1.57 +/- 1.24% vs. 4.38 +/- 2.41%, P < 0.001, n = 20) was significantly lower in ITP patients than healthy controls, at baseline. After one month therapy, the percentage of memory B lymphocytes of ITP patients significantly increased (from 1.66 +/- 1.31% to 3.0 +/- 1.7%, P < 0.009, n = 17). The initial value of regulatory T (0.33 +/- 0.30%, n = 10 vs. 0.16 +/- 0.05%, n = 7, P > 0.05) and memory B lymphocytes percentages (2.1 +/- 1.8%, n = 10 vs. 1.1 +/- 0.75%, n = 7, P > 0.05) were not significantly different for those who had complete response to first line therapy than those required splenectomy. These results indicate that regulatory T lymphocytes and memory B lymphocytes percentages are not useful for predicting treatment outcome in patients with newly diagnosed adult patients with ITP.