Recent reports suggest that intravenous gammaglobulin (IVG) may be an effective treatment modality in patients with immune thrombocytopenia (ITP). Two newborns with isoimmune thrombocytopenia secondary to HLA-A2 and PLA1 platelet antigen incompatibilities with their respective mothers and two newborns with thrombocytopenia secondary to maternal ITP were treated with IVG 400 mg/kg/day x 5 days. One patient was exposed to steroids in utero; only one mother was thrombocytopenic at the time of delivery. All patients were severely thrombocytopenic on day 1 of treatment with mean platelet count of 5.7 x 10(9)/L. All had petechiae and positive quaiac stools, and patients with isoimmune thrombocytopenia had CT scan evidence of intracranial bleeds. The mean platelet count after 24 hr was 26.7 x 10(9)/L and the average platelet increase was 21 x 10(9)/L and 33 x 10(9)/L at 24 and 48 hr, respectively. The two cases with isoimmune thrombocytopenia had sustained platelet increases; the two cases secondary to maternal ITP had transient platelet elevations. IVG can rapidly elevate the platelet count in these patients, especially those with severe bleeding manifestations.