The current first-line choice of treatment of idiopathic thrombocytopenic purpura (ITP) in adults, prednisone, is effective but has many side effects. Furthermore, reduction of the dose leads to a relapse of ITP in a majority of cases. Courses of high-dose dexamethasone (HD) aim to avoid these problems. We treated 36 patients with newly diagnosed or recurrent ITP with an 8-day course of HD, with a peak dose of 40 mg/day. The courses were repeated up to a maximum of six courses, with a 28-day interval. Acute and chronic effects of HD on platelet counts were observed, as well as side effects. HD led to an acute response (rise of platelet count to a level above 50 x 10(9)/l) in 83%. When HD was given as a first-line treatment, 59% of patients were still in remission after 31 months. When HD was given as a second-line treatment, 50% of patients were in remission after 5 months, declining to 25% after 54 months. Side effects were frequent but rarely dangerous. In conclusion, acute effects of HD were excellent. Long-term effects of HD as a first-line therapy of ITP were good, but its long-term effects as a second-line therapy were much poorer.