All patients receive SQV as monotherapy or in combination with other antiretroviral drugs chosen by the investigator and patient. (This study provides only SQV.) Patients may not choose another protease inhibitor (PI) as part of their combination therapy; furthermore, patients taking PIs other than SQV must undergo a 5-day washout period before study-drug administration. Patients are monitored for adverse events and changes in routine laboratory tests, plasma viral load, and CD4+ T cell levels. Outcomes are analyzed by stratifying patients according to CD4+ T cell levels and prior history with protease inhibitors.