The Viral Activation Transfusion Study compared leukocyte-reduced to unfiltered red blood cell transfusions in human immunodeficiency virus (HIV)- and cytomegalovirus (CMV)-coinfected patients. Relationships between serially measured plasma CMV load and clinical and laboratory outcomes over a median of 12 months were examined in 511 subjects. At baseline, subjects had a median of 15 CD4(+) cells/mm(3), 25% had CMV disease, and 21.5% were viremic. No relationship was found between changes in CMV viremia and changes in HIV RNA. Increased CMV viremia was associated with a concomitant fall in Karnofsky score. Highly active antiretroviral therapy (HAART) led to a decrease in CMV viremia after a 90-day delay. After adjustment for HIV load and CD4(+) cell count, CMV viremia remained associated with an increased risk of CMV disease (relative hazard, 5.78). In late-stage HIV-infected patients, CMV viremia was associated with lower functional status and increased risk of CMV disease. HAART suppressed CMV viremia only after a delay of several months.