Prior to study entry patients are stratified according to HIV-1 RNA level, CD4 cell count, and number of antiretroviral drugs. PMPA Prodrug or placebo is added to current antiretroviral regimens, and is administered in one of three dosing regimens. Patients randomized to receive placebo are eligible to receive open-label PMPA Prodrug for the remainder of the 48-week study period after at least 24 weeks post randomization.