Gammaglobulin treatment was given at a dose of 1 gm/kg/day intravenously in 29 patients with acute idiopathic thrombocytopenic purpura: 15 previously untreated, 10 resistant to steroids, and four who were steroid dependent. The average platelet increase in 24 hours was greater than 50,000/microliter; the average peak platelet count was 194,000/microliter. Eighteen of 25 patients required only one infusion; 10 of these 18 never required any additional (maintenance) therapy. Outcome in previously untreated and steroid-resistant patients was identical; however, previously untreated patients required only 1.8 gm/kg total dose of gammaglobulin, whereas steroid-resistant patients received 3.9 gm/kg. Only one steroid-dependent child of the 29 patients still requires maintenance therapy, at 6-week intervals. Toxicity was minimal. Cost was minimized by not admitting patients and by giving treatment in one visit, rather than five.