To evaluate the long-term efficacy of high-dose dexamethasone (HD-DXM) treatment for immune thrombocytopenia (ITP), we retrospectively analysed 36 newly diagnosed ITP patients treated with HD-DXM as a first-line treatment. An initial response was obtained in 23 (63.9%) patients, including 11 with a complete response (CR) and 12 with a partial response (PR). Six months after HD-DXM treatment, 26 of 33 (78.8%) evaluable patients achieved objective responses, including 18 CR and 8 PR. Among 13 patients without initial response, very early increased platelet count within a week (VEIP) was observed in 7 patients, 5 (71.4%) of whom achieved a response at 6 months. In 29 patients who had available platelet count within a week, patients showing VEIP revealed longer survival than those who did not (p = 0.026). HD-DXM was an effective treatment for newly diagnosed ITP patients. VEIP after HD-DXM treatment initiation was associated with a long-term objective response in newly diagnosed ITP patients.