The amount of human cytomegalovirus (CMV) DNA in sera is considered to be a direct marker for CMV infection. We established conditions for nested PCR that detected one copy of CMV DNA, and for competitive PCR, which detected five or more copies of CMV DNA quantitatively. We tested 50 microl each of 16 freeze-stored and 5 fresh sera from patients, for CMV DNA. In sera obtained from the same patient at different time points, small amounts of CMV DNA were detected before the onset of CMV pneumonia. In sera from certain CMV-infected patients who were treated with the anti-CMV agent, ganciclovir, CMV DNA was not detected. Quantitative PCR detection of CMV DNA seems to be suitable for predicting early recurrent CMV infection and monitoring the efficacy of antiviral therapy. The qualitative nested PCR examination of CMV DNA in 40 cord blood plasma samples was carried out for the purpose of preventing CMV infection by cord blood stem cell transplantation, and they were all negative for CMV DNA.