OBJECTIVE: To evaluate 24-week virologic effectiveness of novel antiretroviral regimens for treatment of three-class experienced adult patients in a clinical practice setting following the US Food and Drug Administration approval of darunavir (DRV) for this population. DESIGN: A prospective cohort study. SETTING: A single-center, academic HIV clinic. PARTICIPANTS: Three-class antiretroviral-experienced patients changing regimens between July 2006 and May 2008. Sociodemographic, psychosocial, and clinical characteristics were collected at baseline and during prospective follow-up. OUTCOME MEASURES: Plasma HIV viral load below 50 copies/ml and change in CD4 cell count at 24 weeks following regimen change. The Stanford Genotype Database was used to analyze HIV genotype resistance results and determine the number of active drugs in each regimen. Multivariate models and propensity score methods were employed to assess outcome measures. RESULTS: Among 109 three-class experienced patients, who previously received an average of 10.5 prior antiretrovirals, 55% achieved viral load below 50 copies/ml at 24 weeks. Treatment strategy was classified as nonprotease inhibitor (n = 25), DRV/ritonavir (DRV/r) (n = 51), or other protease inhibitor (n = 33). The number of active drugs was not significantly different across strategies (P = 0.24). In multivariate analysis, patients treated with DRV/r (65%, odds ratio = 4.24 vs. nonprotease inhibitor strategy, 95% confidence interval = 1.28-14.06), raltegravir (65%, odds ratio = 3.10, 95% confidence interval = 1.12-8.62), or both were more likely to achieve viral load below 50 copies/ml. CONCLUSION: Among antiretroviral-experienced patients changing regimens, those treated with DRV/r, raltegravir, or both were more likely to achieve a viral load below 50 copies/ml at 24 weeks. The effectiveness of these agents in routine clinical care mirrors their efficacy in clinical trials and has ushered in a new era in the therapy of three-class experienced patients. © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins.