The incidence of cytomegalovirus (CMV) end-organ disease can be reduced in AIDS patients by oral ganciclovir. However, the cost effectiveness of this prophylaxis is low. Targeting prophylaxis to patients with the highest risk of developing CMV disease might be useful. Several studies have shown the potential of various polymerase chain reaction (PCR) assays and of the antigenemia assay to identify the subset of patients with a definitely higher risk of developing CMV disease. We studied the CMV viral load using quantitative PCR in the leukocytes and plasma (or serum) of 28 patients in the four years before they experienced a CMV event. We observed rising CMV DNA copy numbers in the patients' leukocytes a year before the event. In contrast, plasma or serum copy numbers rose later and in fewer patients. In a control population of 21 profoundly immunodeficient patients (median CD4+ T cell count: 31/mm(3)) without history of CMV disease, only five had detectable CMV DNA in the leukocytes, three of whom had barely above-threshold levels. We suggest that, at the present time, leukocyte CMV DNA PCR might represent a sensitive test providing an early warning signal of increased risk of CMV disease. Performing it twice a year might identify patients at risk so that closer monitoring and targeted prophylaxis can be conducted.