This study will assess the safety and efficacy of once-daily administration of Fuzeon compared with twice-daily administration in HIV-1 infected patients who have received prior treatment. Patients will also receive an optimized treatment consisting of antiretroviral (ARV) therapy as determined by the treating physician. The anticipated time on study treatment is 3-12 months, and the target sample size is 100-500 individuals.

Inclusion Criteria:

• HIV-1 infected adults or adolescents >=16 years of age;
• HIV-1 RNA >=5000 copies/mL;
• prior experience or documented resistance to each of the 3 currently available classes of ARV drugs (nucleoside reverse transcriptase inhibitors, non-nucleoside reverse transcriptase inhibitors, and protease inhibitors).

Exclusion Criteria:

• history of prior use of Fuzeon or T-1249;
• female patients who are pregnant or breastfeeding, or who plan to become pregnant during the study;
• current severe illness;
• currently taking drugs affecting the immune system, HIV vaccine, or investigational agents for any conditions other than HIV/AIDS.