The degree and dynamics of cytomegalovirus (CMV) replication were investigated in blood samples that were prospectively collected in the context of a placebo-controlled study evaluating the efficacy of preemptive oral ganciclovir for the prevention of CMV disease after liver transplantation. The degree of viral replication was strongly associated with progression to CMV disease or viremia (risk ratio, 8.8 and 51.5 among patients with virus loads < or =2860 and >2860 copies/10(6) peripheral blood leukocytes, respectively). Preemptive oral ganciclovir therapy diminished the incidence of CMV disease or viremia but did not completely suppress higher levels of CMV replication. Six (21%) of 29 patients had persistent CMV replication during preemptive oral ganciclovir therapy; 2 patients subsequently developed "breakthrough" CMV syndrome. This study identifies a relative cutoff virus load that predicts subsequent development of CMV disease and highlights the inability of oral ganciclovir to suppress CMV replication in a subset of patients.