A group of 466 leukemic bone marrow transplanted patients were reported from 17 European bone marrow transplantation teams. Of these, 285 survived more than 3 months and could be evaluated for chronic GVHD. The cumulative incidence of chronic GVHD was 32% two years after BMT. The following factors were statistically significantly associated with chronic GVHD in bivariate analysis: high donor and recipient age, splenecacute GVHD, pretransplant seropositivity to CMV among the recipients and the donors, and donor seropositivity to 3 or 4 different herpesviruses, compared with 0-2, prior to BMT. In multivariate analysis pretransplant recipient CMV seropositivity in combination with donor CMV seropositivity prior to BMT (P = 0.0006), a previous grade II-IV acute GVHD (P = 0.001), and splenectomy (P = 0.01) were significantly associated with chronic GVHD. Thus, in addition to acute GVHD, CMV immune donor cells may be triggered by latent CMV in the recipient, which may play a role in the triggering of chronic GVHD. The possible role of splenectomy in GVHD is also discussed.