A 34-year-old male patient developed an isolated pericardial relapse of an acute myelogenous leukemia (M3) 11 months after marrow grafting from his HLA-identical brother. Alloenzyme pattern analysis revealed recipient type of the myeloblasts obtained from the pericardial effusion. Recurrence of the original leukemia was preceded by a reactivation of latent cytomegalovirus (CMV) infection which, in spite of a systemic humoral immune response to the virus, persisted in the pericardium as shown by dot-blot hybridization using CMV-specific DNA fragments. Activated T cells propagated with IL-2 from the pericardial effusion did not reveal any cytotoxic or restimulation capacity on the original or relapse myeloblasts, nor on other donor, recipient or NK target cells. Local coincidence of virus persistence and leukemic relapse suggested CMV-mediated modulation of the immune response in the pericardium with consequent induction of a proliferation of the original malignant cell clone. After local chemotherapy and one course of systemic treatment the patient is still in complete remission--longer than after the marrow grafting.