To evaluate the effect of a single course of high dose dexamethasone (HD-DXM) on CD28 and CTLA-4 expression in patients with newly-diagnosed primary immune thrombocytopenia (ITP). Twenty-8 ITP patients (18 females and 10 males, age range 18-65 years, median age 38.5 years) enrolled in this study and 26 healthy volunteers (19 women and 7 men, age range 16-66 years, median age 37 years) served as a control group. The patients were treated with HD-DXM (40 mg/day) for 4 consecutive days. CD28 and CTLA-4 expression was assessed by flow cytometry once-monthly for 6 months. Plasma levels of the cytokines IFN-γ and IL-10 were determined by enzyme-linked immunosorbent assay. One month after treatment, a platelet response was observed in 23 (82%) of the patients. The response rates over the next 5 months were 71%, 57%, 53%, 46%, and 39%, chronologically. We observed a significant decrease in CD28 expression after the first month (34.7 ± 4.8% vs. 44.5 ± 4.4% before treatment), after which the CD28 levels gradually increased. In contrast, CTLA-4 expression increased after the first month (3.2 ± 0.5% vs. 0.8 ± 0.4 before treatment), after which the CTLA-4 levels gradually decreased. Similar dynamic changes were seen in the levels of IFN-γ and IL-10. The dynamic changes of CD28 and CTLA-4 were consistent with those of IFN-γ and IL-10 and with the effectiveness of HD-DXM in the treatment of ITP. Our results suggest that a disturbed CD28/CTLA-4 balance may contribute to the immunopathogenesis of ITP.