The high sensitivity of nested polymerase chain reaction (PCR) offers the possibility of rapid detection of cytomegalovirus (CMV) DNA in serum. Five consecutive serum samples were examined from 52 human immunodeficiency virus (HIV)-seropositive patients (19 of whom had clinically presumed diagnosis of CMV chorioretinitis). Presence of CMV DNA in serum was shown to precede development of clinical disease. Eleven patients who developed chorioretinitis were positive for CMV DNA in serum samples obtained 3 months before clinical disease, and 3 retinitis patients who initially were negative for CMV DNA became positive with the onset of clinical retinitis. In contrast, 29 of 33 HIV-seropositive subjects without clinical CMV chorioretinitis and matched with respect to age and CD4 T cell numbers were negative for CMV DNA in all 5 serum samples. Thus, the presence of CMV DNA in serum analyzed by PCR is a good predictive marker of CMV retinitis in HIV-seropositive subjects. A positive PCR results supports the clinical diagnosis and may be useful for monitoring response to antiviral treatment.