Sixty-eight adults and nine children infected with human immunodeficiency virus type 1 (HIV- 1) were evaluated consecutively for the presence and amount of cell-free infectious virus in their plasma. Viremia was detected in 18 of 68 adults and in five of nine children; titers ranged from 10 to 100,000,000 TCID/ml plasma. Among the adults, none of 19 asymptomatic patients, 4 of 34 AIDS-related complex patients, and 14 of 15 AIDS patients had cell-free infectious virus in their plasma. None of 35 adult subjects with CD4+ lymphocyte counts >400/mm3 were viremic, whereas 3 of 17 with 200-400 CD4+ lymphocytes/mm3 and 15 of 16 individuals with <200 CD4+ lymphocytes/mm3 were plasma viremic. In contrast to adults, each of five children infected with HIV-1 in utero or during the perinatal period were plasma viremic regardless of their CD4+lymphocyte counts (range, 42-2227/mm3), duration of infection, or clinical stage; however, children infected by HIV-1 at older ages were less frequently plasma viremic. Therapy with zidovudine led to a 10- to 106-fold decline in plasma HIV-1 TCID in all eight subjects studied before and after treatment. © 1991 by The University of Chicago.