Abstract: Purpose: We studied the safety, tolerability, virologic, and immunologic effects of mycophenolate mofetil (MMF) added to a stable antiretroviral therapy (ART) in the setting of low-level viremia. Methods: MMF 500 mg BID or placebo was given to patients thought to be adherent on stable ART with plasma viremia between 200 and 4000 copies/mL. At week 4 unblinding was performed and patients on placebo were offered open-label MMF. Results: Six patients were enrolled. At entry mean plasma HIV-1 RNA (VL) was 2.98 log10 copies/ mL; mean CD4 count was 523. All subjects randomized to placebo elected to cross over to open label MMF. No significant adverse events were observed during MMF therapy. Three patients on MMF achieved VL < 50 copies/mL by week 4; a fourth had VL decline of > 0.5 log. Two patients on placebo had declines of VL. One of these had further decline on open label MMF. Cell surface markers of apoptosis, activation, and proliferation on CD4+ and CD8+ cells declined modestly or remained low. CD4 counts were stable at week 24. All but one subject had rebound of viremia by week 24, universally associated with missed doses of medication by pill count. Conclusion: MMF appears to be safe, and its administration lead to decreased T cell activation. During periods of adherence to therapy, the use of MMF was correlated with declines in viremia, but this small pilot study could not prove this association. Further study of MMF in patients with viremia should be considered for whom additional or alternative antiretrovirals are impractical.