Resistance to antiretrovirals continues to be a major problem for patients with HIV infection. A number of studies presented recently at the 1st International AIDS Society Conference on HIV Pathogenesis and Treatment [Buenos Aires, Argentina; July 2001] dealt with this complex issue. In the first study, researchers evaluated genotypic resistance to lopinavir/ritonavir and nelfinavir in previously untreated patients with HIV infection, and found higher levels of resistance to nelfinavir than to lopinavir/ritonavir. Another team of investigators noted that non-nucleoside reverse transcriptase inhibitor (NNRTI) resistance mutations appear to persist in several patients after withdrawal of this class of drugs. Two additional studies reported that, compared with standard of care, phenotypic or genotypic testing did not significantly improve response to further therapy in patients failing a protease inhibitor-based regimen.