Cytomegalovirus (CMV) has been a major cause of morbidity and mortality after allogeneic stem cell transplantation (SCT). The importance of the recipient's serologic status is paramount. However, the importance of the donor's serologic status in CMV-seropositive recipients is controversial. We analyzed the influence of the donor's CMV status in a large cohort of patients. A total of 7018 patients seropositive for CMV reported to the European Group for Blood and Marrow Transplantation (EBMT) were included; 5910 patients had undergone HLA-identical sibling SCT and 1108 patients had undergone unrelated donor SCT. Univariate and multivariate proportional hazards models were constructed for survival, event-free survival, transplant-related mortality, and relapse incidence. Patients receiving grafts from CMV-seropositive HLA-identical sibling donors had the same survival as patients grafted from seronegative donors (hazard ratio [HR], 1.04; P = .37; 95% confidence interval [CI], 0.95-1.14). However, unrelated donor stem cell (SC) transplant recipients receiving grafts from CMV-seropositive donors had an improved 5-year survival (35% versus 27%; HR = 0.8; P = .006), an improved event-free survival (30% versus 22%; HR = 0.8; P = .01), and a reduced transplant-related mortality (49% versus 62%; HR = 0.7; P < .001). There was no influence on the relapse incidence. The effects of donor CMV status remained in multivariate analyses. The effect of donor status was different among different disease categories. In patients with chronic myelogenous leukemia (CML), T-cell depletion abrogated the beneficial effect of donor status, suggesting that the effect is mediated through transfer of donor immunity. Our data suggest that donor CMV status influences outcome of unrelated SCT. For a CMV-seropositive patient, a seropositive donor might be preferable. (Blood. 2003;102:4255-4260)