Eligible patients remain on their pre-study regimen until baseline. An OB regimen is chosen by the physician and patient based on the patient's prior treatment history, prior and current laboratory abnormalities, the screening GT/PT antiretroviral resistance testing, and any prior GT/PT antiretroviral resistance (if available). The drugs in the OB regimen are chosen from among the currently approved antiretrovirals and permitted newly approved/investigational antiretrovirals available in the countries where the study is implemented, and must consist of 3 to 5 drugs, including no more than 1 newly approved/investigational agent. Patients are stratified with respect to viral load and use (versus non-use) of any of the allowed newly approved/investigational antiretrovirals. Patients are randomized to receive 1 of the following 2 treatments for 48 weeks: OB or OB plus T-20. Patients are followed to assess viral load, safety, antiretroviral resistance, T-20 pharmacokinetics, and quality of life. At the end of 48 weeks of treatment patients are allowed to (a) roll over and receive OB plus T-20 (for patients receiving OB regimen alone) or (b) continue taking OB plus T-20 (for patients already receiving OB plus T-20), for an additional 48 weeks (plus 4 weeks safety follow-up period), or until 12 weeks after commercial availability of T-20 in the country in which they are treated, whichever comes first. All patients are followed in this study for a maximum of 100 weeks from their initial baseline visit date.