To determine whether human immunodeficiency virus (HIV) viral load has short term stability, eight clinically stable subjects infected with HIV and having CD4 counts ranging between 10-600/mm3, had blood samples taken at 0800 and 1700 on 3 consecutive days and then weekly at 0800 for 1 month (8-10 observations/subject). Plasma HIV RNA, peripheral blood mononuclear cell (PBMC) proviral DNA, serum p24 antigen levels, and mononuclear cell subsets were measured at each time point. Mean plasma HIV RNA, PBMC HIV DNA, and p24 antigen [both regular and immune complex dissociated (ICD)] levels did not change significantly between mornings and afternoons or on successive days or weeks. CD4+, CD8+, and CD56+ number demonstrated a diurnal variation in those subjects with > 200 CD4 cells/mm3. We conclude that HIV viral load demonstrates short-term stability in clinically stable subjects. This stability has important implications for monitoring HIV disease progression or antiretroviral therapy.