Forty-five patients at high risk for cytomegalovirus (CMV) infection were studied during the first 100 days after marrow transplant to determine the relationship between cytotoxicity against CMV-infected HLA-mismatched target cells and CMV infection. Peripheral blood lymphocytes (PBL) from 35 patients who developed CMV infection had significantly lower cytotoxicity against CMV-infected targets 20–40 days transplant compared with PBL from normal controls (P=0.003). Responses by PBL from 10 uninfected transplant recipients were not significantly decreased. The PBL that lysed CMV targets had phenotypic and Erosetting characteristics consistent with natural killer cells. Cytotoxicity against CMV-infected targets by PBL from patients with acute graft-versus-host disease (GVHD) was reduced 20–40 days after transplant compared with responses by patients without GVHD (P=0.04). Survival from CMV infection was longer in patients whose PBL had >15% lysis of CMV-infected targets during the first 20–60 days after transplant (P=0.04). This study suggests that natural cytotoxicity of CMV targets is an important correlate of the acquisition and outcome of CMV infection after marrow transplant.