The influence of pretransplant herpes-virus antibodies in 126 marrow-graft recipients and their HLA-identical (A, B, C, DR) sibling donors on the incidence of grades II-IV acute graft-versus-host disease (GVHD) was studied in relation to previously reported risk factors for GVHD. Logistic regression procedures were used to control for confounding factors. Increasing donor age (odds ratio 3.7 per decade; p = 0.02) and donor seronegativity for Epstein-Barr virus (EBV; odds ratio 10.1; p = 0.005) were associated with a high incidence of GVHD. Total rather than selective gastrointestinal decontamination (odds ratio 0.1; p = 0.004), donor seronegativity for herpes simplex virus (HSV; odds ratio 0.1; p = 0.003), and recipient EBV-seronegativity (odds ratio 0.05; p = 0.002) were associated with a low incidence of GVHD. Pretransplant EBV and HSV serology may thus contribute substantially to the estimation of the risk for GVHD.