We analysed 35 risk factors for acute GVHD in 291 consecutive recipients of HLA-identical sibling marrow transplants from 1975 to 1993. Of these, 16% developed moderate-to-severe acute GVHD following transplantation. In multivariate analysis, GVHD prophylaxis with monotherapy (MTX or CsA)(P= 0.015) seropositivity for several herpes viruses in the donor (P= 0.015) and seropositivity for CMV in the recipient (P= 0.037) before the transplants as well as early engraftment (P= 0.016), were the principal risk factors for GVHD. A high serum TNF-alpha level during conditioning therapy was also a significant risk factor in 75 recipients (P= 0.005). The risk of grades II-IV acute GVHD increased with the number of risk factors. Thus the cumulative incidence of acute GVHD was 1%, if no risk factor was present, 4% with one, 9% with two, 21% with three and 44% in patients with four risk factors. Factors reported to correlate with acute GVHD, such as age, diagnosis, female donor to male recipient, relative response and donor-responding capacity in MLC, MNS blood group antigen, splenectomy and bone marrow cell dose were not associated with acute GVHD in this study. Five-year survival was 24% in patients with grades II-IV GVHD vs 62% in patients with grades 0-I GVHD (P= 0.0001).