Intravenous immunoglobulin is approved for use in allogeneic bone marrow transplant recipients for prevention of graft-versus-host disease (GVHD) and infections, but the minimally effective dose has not been established. In this multicenter, randomized, double-blind trial, patients undergoing allogeneic marrow transplantation were randomized to receive 100 mg/kg, 250 mg/kg, or 500 mg/kg doses of intravenous immunoglobulin. Each dose was given weekly for 90 days and then monthly until 1 year after transplant. Six hundred and eighteen patients were evaluated. Acute GVHD (grades 2-4) occurred in 39% of the patients (80 of 206) in the 100 mg/kg group, 42% of the patients (88 of 208) in the 250 mg/kg group, and in 35% of the patients (72 of 204) in the 500 mg/kg group (P = 0.344). Among patients with unrelated marrow donors, a higher dose of intravenous immunoglobulin (500 mg/kg) was associated with less acute GVHD (P = 0.07). The incidences of chronic GVHD, infection and interstitial pneumonia were similar for all three doses of intravenous immunoglobulin. The dose of intravenous immunoglobulin also had no effect on the types of infection, relapse of hematological malignancy or survival. Except for more frequent chills (P = 0.007) and headaches (P = 0.015) in patients given the 500 mg/kg or 250 mg/kg dose of immunoglobulin, adverse events were similar for all three doses. These results suggest that 100 mg/kg, 250 mg/kg, and 500 mg/kg doses of intravenous immunoglobulin are associated with similar incidences of GVHD and infections in most allogeneic marrow transplants. These results should be considered when designing cost-effective strategies for the use of intravenous immunoglobulin in allogeneic marrow transplants receiving other current regimens for prophylaxis of GVHD and infection.