BACKGROUND: The long-term effectiveness of potent three-drug antiretroviral regimens for the treatment of human immunodeficiency virus type 1 (HIV-1) infection is limited by problems related to compliance and tolerability. We investigated whether two-drug maintenance therapy would suppress viral replication after a three-month period of aggressive triple-drug induction therapy. METHODS: A total of 378 HIV-1-infected adults who had not received previous antiretroviral treatment received three months of induction therapy consisting of 300 mg of zidovudine every 12 hours, 150 mg of lamivudine every 12 hours, and 800 mg of indinavir every 8 hours. The 279 patients in whom the plasma HIV-1 RNA titer fell below 500 copies per milliliter after two months of triple-drug therapy, and who completed the induction phase, were randomly assigned at month 3 to one of the following three open-label maintenance regimens: zidovudine, lamivudine, and indinavir; zidovudine and lamivudine; or zidovudine and indinavir. The primary end point was an increase in HIV-1 RNA levels to 500 copies or more per milliliter during the maintenance phase. RESULTS: The proportion of patients who reached the primary end point was significantly higher among patients receiving zidovudine plus lamivudine (29 of 93 patients, P<0.001) or zidovudine plus indinavir (21 of 94, P=0.01) than among patients receiving continued triple-drug therapy (8 of 92). This higher failure rate in the groups treated with the two-drug maintenance regimens was also observed in the subgroup of patients with maximally suppressed HIV-1 RNA (below 50 copies per milliliter) at the time of randomization to maintenance therapy. CONCLUSIONS: In HIV-1-infected adults not previously treated with antiretroviral drugs whose plasma HIV-1 RNA levels fell below 500 copies per milliliter after three months of induction therapy with zidovudine, lamivudine, and indinavir, two-drug maintenance therapy was less effective in sustaining a reduced viral load than continued three-drug therapy.