The purpose of this study is to see if it is safe and effective to add PMPA Prodrug (a new anti-HIV drug) to an anti-HIV drug combination taken by patients who have taken anti-HIV drugs in the past. Genetic response will be studied.

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.

Inclusion Criteria

You may be eligible for this study if you:

• Are HIV-positive.
• Have an HIV count of 400 - 50,000 copies/ml.
• Are expected to live for at least 1 year.
• Have been taking the same anti-HIV drug combination (made up of no more than 3 anti-HIV drugs) for at least 8 weeks prior to study entry.
• Are at least 18 years old.
• Agree to use effective methods of birth control during the study.

Exclusion Criteria

You will not be eligible for this study if you:

• Have taken medications for certain infections within 15 days prior to study entry.
• Have a history of cancer, except Kaposi's sarcoma (KS) or skin cancer.
• Develop a new AIDS-related condition within 30 days of study entry.
• Have certain serious medical conditions, including severe nausea, vomiting, or other stomach problems that make you unable to take medications by mouth.
• Have received a vaccine within 30 days prior to study entry.
• Have taken certain medications, including those that may affect your kidneys.
• Abuse alcohol or drugs.
• Are pregnant.