Cytomegalovirus (CMV) infection is an important cause of disease in immunocompromised patients. In a prospective longitudinal study of 34 septic patients, the incidence of active CMV infection was examined. Eleven of 34 patients (32.4%) had active CMV infection, diagnosed by immunocytochemical staining of CMV pp65 antigen in blood leukocytes and/or detection of CMV DNA by PCR. Positive results for CMV infection were obtained in a median of 4 days (by PCR) or 11 days (by staining of pp65 antigen) after onset of sepsis. Twenty patients for whom more than one sample was examined were selected for further analysis. Among the patients with active CMV infection (nine of 20) there was a trend toward higher median values of tumor necrosis factor alpha, interleukin-1 beta, alanine aminotransferase, and aspartate aminotransferase in plasma, in comparison with the values for patients without CMV infection. Sepsis in patients with CMV infection may affect outcome of the disease.