Short-term variability in CD4 cell counts, plasma RNA levels, and p24 antigenemia was measured in clinically stable men with human immunodeficiency virus infection. Blood samples were obtained three times in each of 2 weeks 4 weeks apart. Differences between duplicate assays were considered to be due to technical factors; variation among visits was attributed to physiologic factors. Median enrollment values of CD4 cell counts, plasma RNA levels, and immune complex-dissociated (ICD) p24 antigenemia were 400 cells/mm3, 3.4 log10 copies/mL, and 38 pg/mL, respectively. Median absolute differences between duplicate CD4 cell counts, plasma RNA levels, and ICD p24 antigen were 16 cells/mm3, 0.21 log10 copies/mL, and 9.3 pg/mL. Median intraindividual ranges were 119 cells/mm3, 0.83 log10 copies/mL, and 18 pg/mL. Above 500 copies/mL, plasma RNA levels have a variability of 0.5 log10; the test is unreliable below this level.