. The efficacy of Vigam-S, a highly purified intravenous immunoglobulin, was investigated by an open, noncomparative study in 20 adults with chronic idiopathic thrombocytopenic purpura (ITP). Fifteen patients responded to the initial 3-day infusion of 0·4 g kg −1 day−1 by exhibiting an incremental increase in platelet count of ≥ 30 × 109 L−1, in eight of whom platelet count normalized (> 150 × 109 L−1). The peak platelet count for responders on day 4 was 163 × 109 L−1 (baseline = 18 × 109 L−1). No benefit was derived from an extra 2 days infusion in nonresponders. Further treatment (either a single 0·8 g kg−1 dose or another 3-day infusion) given to responders when platelet counts fell below 30 × 109 L−1 was effective on eight of 14 occasions. Increases in total serum IgG concentration (to a mean peak of 25·3 g L−1) were not correlated with platelet response. There was no evidence of seroconversion to virus markers, or of alteration in renal function, following Vigam-S infusion. Most adverse events were mild and transient; however, three patients had severe headache and vomiting (possible aseptic meningitis syndrome) and one had marked superficial thrombophlebitis. Therefore Vigam-S provides effective and well tolerated therapy in the management of adults with ITP although individual patient response remains difficult to predict.