Nine newly diagnosed, previously untreated children (mean age: 4.2 years, range: 1-9 years) with severe acute idiopathic thrombocytopenia purpura (mean platelet count: 5.8 X 10(9)/L, range: 1-12 X 10(9)/L) were treated with high-dose steroids (prednisone 4-8 mg/kg/day). Steroid dose was based on platelet count at presentation: Group I (platelets less than 5 X 10(9)/L) was started on 8 mg/kg/day; Group II (platelets 5-15 X 10(9)/L) received 6 mg/kg/day. All patients had serologic and histologic evidence of acute idiopathic thrombocytopenia purpura. On this protocol, it took a mean number of 1.9 days (1-3 days) to reach a platelet count of at least 20 X 10(9)/L and 9.2 days (3-26 days) to reach a normal platelet count. No significant toxicity was observed except for weight gain ranging from 3-10% and mild behavioral problems. Both groups were on high-dose steroids (4-8 mg/kg/day) for 7.3 +/- 2.1 days. Only one patient had a brief relapse to a platelet count of 18 X 10(9)/L while on therapy (day 14), but responded promptly to an increase in prednisone dose. Presently, all nine patients are in remission and have not required maintenance medication.