Human cytomegalovirus causes severe and often fatal infections in immunocompromised patients. After organ transplantation cytomegalovirus in peripheral blood mononuclear cells is thought to be activated by alloreaction and to spread because of immunosuppression, and it may cause endogenous cytomegalovirus diseases. Patients with cirrhosis, one group of candidates for liver transplantation, often show various grades of immunosuppression before transplantation. To evaluate the status of cytomegalovirus infection in cirrhotic patients and its relevance to the degree of immunosuppression, we examined the presence of cytomegalovirus in mononuclear cells by polymerase chain reaction and immunocytochemical analysis. We studied 122 patients with definite cirrhosis and 43 normal volunteers. All cirrhotic patients (100%) and 40 (93%) of 43 normal controls were seropositive for cytomegalovirus. Cytomegalovirus DNA was detected by polymerase chain reaction in 77 (63.1%) of 122 seropositive cirrhotic patients, but in only 1 (2.5%) of 40 seropositive normal controls (p < 0.01). Cytomegalovirus antigen could not be detected in mononuclear cells by immunocytochemical staining with monoclonal antibodies. Cytomegalovirus DNA—positive patients have a greater impairment of liver function than do cytomegalovirus DNA—negative patients; this fact is manifested by delayed indocyanine green retention rates and elevated serum bilirubin levels (p < 0.05). Lymphocyte proliferative response induced by phytohemagglutinin and natural killer cell activity were also significantly lower in cytomegalovirus DNA—positive patients as compared with cytomegalovirus DNA—negative patients (p < 0.01). Our data suggest that the reactivation of cytomegalovirus may have already occurred in patients with cirrhosis before transplantation.