In a placebo-controlled, randomized blinded study, we evaluated the efficacy of intravenous gammaglobulin (IV-IgG) in alloimmunized thrombocytopenic patients. IV-IgG was administered at a dose of 400 mg/kg for 5 days. An incompatible platelet transfusion from the same donor was used before and after treatment. Seven patients received IV- IgG and five patients received placebo. Although platelet recovery in 1 to 6 hours was satisfactory in five patients after IV-IgG treatment, 24- hour survival was not improved in most patients. None of the patients receiving the placebo achieved satisfactory 1-hour platelet-corrected count increments (CCIs). By t test, the posttreatment mean values 1 hour after transfusion CCIs in the IV-IgG group were significantly greater than in the control group (8,413 v 1,050, P less than .007). Using a regression model to adjust for any distributional assumptions of the study population, the parameter estimate for IV-IgG treatment was positive, indicating that IV-IgG treatment is associated with higher CCIs. Although IV-IgG may improve 1-hour platelet recovery, clinical benefit was not demonstrated since 24-hour survival was not improved. IV-IgG treatment before unmatched platelet transfusions should not be considered as a replacement for HLA-compatible platelets in alloimmunized patients.