Enfuvirtide is the first fusion and entry inhibitor approved for use for the treatment of human immunodeficiency virus (HIV) type 1 infection and as such represents a novel class of agents. For the population of patients experienced with three antiretroviral classes, enfuvirtide provides an additional option for treatment. This prospective, open-label, 24-week, single-arm trial assessed the efficacy and safety of enfuvirtide (90 mg injected subcutaneously twice daily) in combination with darunavir-ritonavir (600/100 mg administered orally twice daily) in triple-antiretroviral-class-experienced adults failing their current regimen. The primary efficacy endpoint was the proportion of participants with plasma HIV RNA loads of <50 copies/ml. Other virological and immunological measures were also evaluated, as were the effects of the baseline viral coreceptor tropism and darunavir phenotype sensitivity scores on the outcomes. At week 24, 60.3%, 72.5%, and 84.0% of 131 participants achieved viral loads of <50 copies/ml and <400 copies/ml and a change from the baseline load of > or =1 log(10) copies/ml, respectively. A baseline viral load of < or =5 log(10) copies/ml was a significant predictor of achieving a viral load of <50 copies/ml at 24 weeks; however, neither background genotype sensitivity nor darunavir phenotype sensitivity was a significant predictor of the achievement of viral loads of <50 copies/ml. Although these findings are limited by the relatively small numbers of participants with darunavir susceptibility changes of > or =10-fold, they suggest that combining enfuvirtide and darunavir-ritonavir with an optimized background regimen in triple-class experienced participants naïve to these agents can result in positive virological and immunological responses regardless of most baseline parameters.