Objective: Immune thrombocytopenia (ITP) is an autoimmune disease, and it has become evident that T lymphocytes play an important role in the pathogenesis of ITP. We investigated the role of T helper (Th) intracellular IL-2, IL-4, IL-6, IFN-gamma, and T lymphocyte apoptosis in the pathogenesis of acute ITP and the effect of glucocorticoid treatment on cytokine profile. We investigated also P-glycoprotein (P-gp) and glucocorticoid receptor (GCR) expression as a possible mechanism for glucocorticoid resistance. Material and Methods: The study includes 20 children with acute ITP having a platelet count <20,000/mm(3) and 20 healthy children as a control group. Patients with acute ITP were treated with megadose methylprednisolone (MDMP) (MDMP in the dose of 30 mg/kg/d between day 1 and 3 and 20 mg/kg/d between day 4 and 7). Th intracellular IL2, IL-4, IL-6, and IFN-gamma percentages, T-cell P-gp expression, T-cell and monocyte GCR expression, and T-cell apoptosis were evaluated before and after treatment in acute ITP patients and in the control group. Results: Acute ITP patients had significantly higher Th IL-2, IL-4, IL-6, and IFN-gamma percentages compared with the control group (P<0.05). Th IL-2 and IFN-gamma percentages were significantly lowered with MDMP treatment (P<0.05). IFN-gamma/IL-4 ratio was also lowered with the MDMP treatment (P<0.05). T-lymphocyte P-gp expression and T lymphocyte and monocyte GCR expression were all similar between acute ITP pretreatment and control groups (P>0.05). T-lymphocyte P-gp expression was higher in the posttreatment group than in the pretreatment group (P<0.05). Both T lymphocyte and monocyte GCR expression percentages were not different in the pretreatment and posttreatment groups (P>0.05). Early apoptosis in T lymphocytes was significantly lower in the pretreatment acute ITP group than in the control group (P<0.05). Necrotic apoptosis in T lymphocytes was significantly increased with MDMP treatment (P<0.05). Conclusions: Th1 and Th2 cytokine profile is observed in acute ITP pathogenesis, and MDMP treatment causes Th1 to Th2 cytokine profile shift and induction of T-lymphocyte apoptosis. There is a need to have a greater number of resistant cases in order to better evaluate the P-gp and GCR expression in glucocorticoid resistance in acute ITP.