BACKGROUND: The new protease inhibitors are potent inhibitors of the human immunodeficiency virus (HIV), and in combination with other antiretroviral drugs they may be able to cause profound and sustained suppression of HIV replication. METHODS: In this double-blind study, 97 HIV-infected patients who had received zidovudine treatment for at least 6 months and had 50 to 400 CD4 cells per cubic millimeter and at least 20,000 copies of HIV RNA per milliliter were randomly assigned to one of three treatments for up to 52 weeks: 800 mg of indinavir every eight hours; 200 mg of zidovudine every eight hours combined with 150 mg of lamivudine twice daily; or all three drugs. The patients were followed to monitor the occurrence of adverse events and changes in viral load and CD4 cell counts. RESULTS: The decrease in HIV RNA over the first 24 weeks was greater in the three-drug group than in the other groups (P<0.001 for each comparison). RNA levels decreased to less than 500 copies per milliliter at week 24 in 28 of 31 patients in the three-drug group (90 percent), 12 of 28 patients in the indinavir group (43 percent), and none of 30 patients in the zidovudine-lamivudine group. The increase in CD4 cell counts over the first 24 weeks was greater in the two groups receiving indinavir than in the zidovudine-lamivudine group (P< or =0.01 for each comparison). The changes in the viral load and the CD4 cell count persisted for up to 52 weeks. All the regimens were generally well tolerated. CONCLUSIONS: In most HIV-infected patients with prior antiretroviral therapy, the combination of indinavir, zidovudine, and lamivudine reduces levels of HIV RNA to less than 500 copies per milliliter for as long as one year.