The purpose of the study was to determine long-term efficacy, safety, and tolerability of atazanavir plus stavudine/lamivudine in 346 HIV-infected patients previously treated with atazanavir or nelfinavir. BMS AI424-044 is an ongoing, multicenter, international, open-label, rollover/switch study initiated in June 2001. Patients completing >or=48 weeks in trial BMS AI424-008 with a plasma HIV RNA viral load <10,000 copies/mL were eligible to continue on atazanavir (400 or 600 mg) or to switch from nelfinavir to atazanavir (400 mg) once daily. Antiviral efficacy, change in CD4 cell counts, and effect on lipid parameters were measured. After 24 weeks of atazanavir use in BMS AI424-044, 83%, 85%, and 87% of the atazanavir 400-mg, atazanavir 600-mg, and nelfinavir-to-atazanavir-switched patients, respectively, had HIV RNA levels <400 copies/mL compared with 76%, 76%, and 63%, respectively, at week 48 of BMS AI424-008. Atazanavir-treated patients showed minimal changes in lipid levels compared with baseline. Patients switched from nelfinavir to atazanavir showed significant mean percent decreases in total cholesterol (-16%), fasting low-density lipoprotein cholesterol (-21%), and fasting triglycerides (-28%) (P<0.0001) by week 12 of atazanavir treatment. No new safety issues were identified, and the overall incidence of treatment-emergent adverse events during BMS AI424-044 was comparable across treatment groups. Atazanavir was safe, tolerable, and effective during extended use and in patients switched from nelfinavir. Extended atazanavir use resulted in continued viral suppression and lipid changes that were not clinically relevant. In virologically suppressed nelfinavir-treated patients switched to atazanavir, virologic improvement continued, whereas nelfinavir-induced lipid elevations were reversed within 12 weeks, approaching pretreatment values.