One hundred forty-seven patients with hematologic diseases and treated by allogeneic marrow transplants received graft-versus-host disease (GVHD) prevention with methotrexate and cyclosporine. In addition, 73 of the 147 patients were randomized to receive methylprednisolone during the first 35 days after transplant to improve GVHD prevention, whereas 74 patients were randomized not to receive methylprednisolone. The randomized trial enabled us to examine whether methylprednisolone increased the risk of infection after marrow grafting. Charts of study patients were analyzed retrospectively for infection events including bacteremia, septicemia, and fungemia. The randomization was stratified by diagnosis, patient age, genotypic HLA identity, and assignment to laminar airflow room isolation. All patients were given a short course of methotrexate (no longer than 11 days) and cyclosporine for no longer than 180 days after marrow transplantation. Methylprednisolone was begun on the day of marrow grafting at a dose of 1 mg/kg body weight intravenously in divided AM and PM doses through day 22. Methylprednisolone was administered at a dose of 0.5 mg/kg in divided doses from days 22 through 35, and then discontinued. Infections were analyzed for the time interval ending on day 65 after transplantation, which included the period of methylprednisolone administration and 1 month thereafter. Seventy-one episodes of first infection events were observed in patients receiving methylprednisolone compared with 47 episodes in patients not receiving the drug. Predominant infections were bacteremias, followed in descending order by fungemias and septicemias. The most prevalent organisms cultured were gram-positive bacteria, especially coagulase-negative Staphylococcus and Streptococcus species. Pseudomonas species were the most common gram negative bacteria, and the most prevalent fungus was Candida albicans. Multivariable Cox regression analysis showed that patients receiving methylprednisolone had a 1.5 times higher risk of infection (P = .03), with acute GVHD being another independent risk factor for infections (P = .005). Methylprednisolone, when added to GVHD prevention by methotrexate and cyclosporine, increases the risk of infection during the early posttransplantation period.