We compared the value of PCR on plasma with PCR on buffy coat leukocytes, Ag assay, and the determination of IgM antibodies by ELISA for the diagnosis and follow-up of cytomegalovirus infection. Thirty patients were followed after liver transplantation (LTX). We compared the tests to assess their clinical usefulness. Fourteen of 30 (46%) patients were both positive in plasma and buffy coat PCR and Ag test. Sixteen patients were negative in both procedures. There was a 97.2% concordance between PCRs done from plasma or buffy coat. The concordance of results of PCR and Ag test in single samples was 94.3%. Discordant results were found in 5.6% of samples. Discordance was observed in the early and the late phase of CMV infection and was due to positive PCRs preceding positive Ag tests for 1-3 weeks in one-half of the patients. IgM antibodies were first observed after a median period of 8 weeks (range, 6-11 weeks) after LTX. Positive PCRs and Ag tests preceded clinical manifestation of CMV disease by a 1 week median (range, 0-3 weeks), whereas positive IgM ELISAs occurred after a median period of 2.5 weeks (range, 0-4 weeks) after the onset of CMV disease. The sensitivity and specificity of both PCR and Ag test were identical, 100% and 76%, respectively. However, for the IgM ELISA, the sensitivity was only 66%, and the specificity was 84%. In conclusion, plasma or buffy coat PCR and Ag test are equally reliable procedures for early detection and monitoring of CMV infection. PCR can become positive earlier than the Ag test, but it is technically more demanding to perform. The demonstration of IgM antibodies is of little practical help because an antibody response occurs too late in relation to infection.