For early diagnosis of cytomegalovirus (CMV) infection after renal transplantation, 18 patients were monitored within the first 3 months. Blood leukocytes were assayed weekly for the presence of the CMV matrix protein p65 (antigenemia assay) and CMV DNA by the polymerase chain reaction (PCR). In 12 out of 18 patients, 70 (37.8%) of 185 blood samples were positive by antigenemia assay or by PCR. 49 and 115 samples concurrently were positive and negative by both tests. 8 blood samples were positive only by the antigenemia assay and 13 only by PCR. Therefore, the relative sensitivity of PCR and of antigenemia assay was 88.6% and 81.4% respectively. In 9 of 10 patients who were pre-transplant CMV seropositive and in 4 of 7 patients who were pre-transplant CMV seronegative and received a graft from a seropositive donor, CMV infection was detected by positive antigenemia and/or PCR. Four of these patients developed CMV disease. In all these patients, antigenemia assay and PCR were positive 7 to 16 days prior to onset of clinical symptoms. Antigenemia and PCR are rapid and sensitive methods for the detection of CMV infection 1 to 2 weeks prior to disease. In addition, the semi-quantitative assay of antigenemia enables monitoring of the efficacy of antiviral therapy.