We analysed the genetic changes in gp41 in a population of 30 heavily pretreated HIV-1-infected patients failing on a T-20-containing salvage therapy. This study confirms the key role of the number of drugs presumed to be still effective associated with T-20 in the magnitude of the virological response. Our results suggest that only HR-1 sequencing is necessary to characterize resistance to T-20, and that HR-2 domain sequencing is probably not required.