Human cytomegalovirus (CMV) DNA copy number in white blood cells from both human immunodeficiency virus (HIV)-seronegative and HIV-seropositive patients was amplified from the immediate-early region of CMV DNA and quantified by colorimetric detection of the hybridization of the amplification product to a detector oligonucleotide probe in microtiter wells. By Mann-Whitney U test, significantly higher (P < .05, two-tailed) copy numbers of CMV DNA were detected in HIV-seropositive patients with retinitis than in either patients with < 100 CD4 cells/mm3 and no symptomatic CMV disease or HIV-seropositive patients with > 100 CD4 cells/mm3. By prospective monitoring for increases in CMV DNA copy number, it may be possible to identify HIV-seropositive patients who are at imminent risk for development of symptomatic CMV retinitis.