Clinical trials in adults and children suggest that SQV-SGC has advantages over other protease inhibitors in terms of tolerability, safety, and effectiveness in patients who have failed therapy with other protease inhibitors. [AS PER AMENDMENT 11/24/99: Pharmacokinetic data from this study demonstrate lower than expected plasma SQV exposures, particularly in Arm 1. Based on these data, patients in the study will be offered the opportunity to adjust their doses. Ritonavir added to the Arm 1 regimen is expected to significantly increase SQV-SGC plasma levels and allow twice-daily dosing for SQV-SGC.]