This is a prospective, phase IV, multicentre, randomised, open-label, 2-arm, 24-week study. Approximately 130 HIV-1 infected patients on a stable antiretroviral regimen containing Kaletra® with 2 nucleoside/nucleotide analogues will be randomized to 1 of 2 treatment arms: saquinavir with ritonavir 1000/100 mg BID (using Invirase® tablets) or lopinavir/ritonavir 400/100 mg BID (using Kaletra® tablets).Eligibility for enrollment will be determined at a screening visit that will occur within 30 days of the baseline visit. Protocol-defined study assessments will take place at clinic visits at the end of Weeks 4, 12, and 24.