A polymerase chain reaction (PCR) test for CMV DNA was evaluated for clinical usefulness. Leukocytes and serum were sampled from 36 patients who had recently undergone organ transplantation. Clinical symptoms, virus culture, and IgG and IgM antibodies were used to identify, in retrospect, patients with CMV disease certified beyond all doubt, with probable disease, with asymptomatic infection, or without infection. PCR tests for CMV DNA in leukocytes (BC-PCR) and serum (SE-PCR) were then evaluated. BC-PCR was positive in all patients with certified CMV disease but also in 31% of the samples from patients without infection. SE-PCR was positive in 11/13 patients with certified disease and was concordant with CMV culture in 192/231 tests. Of the 39 discordant cases, 27 had a positive SE-PCR with a negative culture. The effect of ganciclovir treatment could not be predicted by any test. In conclusion, a negative BC-PCR is strong evidence against CMV disease and a positive SE-PCR strongly suggests CMV disease, but the opposite results are of little clinical help.