A 48-week open-label study of 11 antiretroviral-naive, human immunodeficiency virus type 1 (HIV-1)-infected adults evaluated once-daily treatment with adefovir dipivoxil, lamivudine, didanosine, and efavirenz. At baseline, the median plasma HIV-1 RNA level was 4.99 log(10) copies/mL, and the median CD4 cell count was 471 cells/mm(3). At 24 and 48 weeks after initiation of treatment, median HIV-1 RNA levels decreased from baseline by 4.77 and 4.99 log(10) copies/mL, respectively, and median CD4 cell counts increased by 135 and 177 cells/mm(3), respectively. The regimen was generally well tolerated. No patients withdrew from the study because of adverse events. However, 7 patients developed adefovir-related nephrotoxicity after >/=20 weeks of treatment; this resolved without sequelae after adefovir was discontinued. Overall adherence was 85%. Once-daily quadruple-drug therapy with adefovir, lamivudine, didanosine, and efavirenz provides pronounced and durable suppression of HIV-1 RNA and elevation of CD4 cell counts over the course of 48 weeks, with generally good tolerability and adherence.