OBJECTIVES:  DUET 1&2 are two identically designed, ongoing, randomized, double-blind, placebo-controlled, phase III trials, which have demonstrated superiority of etravirine (TMC125, ETR) + background regimen (BR) versus placebo+BR in HIV-1-infected, treatment-experienced patients. This analysis estimates the cost of the hospitalizations observed in the DUET studies applied to a Belgian treatment setting. METHODS:  Hospitalization events and duration of hospital stay were recorded for each patient in the DUET trials.  Differences in hospitalizations and total days hospitalized at 48 weeks were compared between ETR+BR and placebo+BR in the pooled DUET population.  Daily hospital costs were collected from the Belgian IMS Hospital Disease database, a longitudinal patient-level retrospective database. A sample of 277 HIV patients with 397 hospitalizations in Belgium were extracted.  An average cost per day was calculated in 2007 Euros, which included INAMI (health system cost) and patient contribution costs for medications, procedures, and rooming charges. These costs were applied to the total days observed in the DUET trials to estimate overall hospital costs. RESULTS: A total of 1203 patients were included from the DUET trials: 599 vs. 604 in the ETR vs. placebo groups. Baseline characteristics and average follow-up were comparable between arms. The number (%) of patients hospitalized was 105 (18%) for ETR+BR vs. 139 (23%) for placebo+BR, respectively (p=0.0006). Total hospital days observed during the 48 week follow-up period were 1702 for ETR+BR vs. 2747 for placebo+BR.  The average cost per patient day was estimated to be €556.39.  Over the population, total hospital costs were estimated to be €946,676 for ETR+BR vs. €1,528,403 for placebo+BR. CONCLUSIONS: At Week 48, ETR+BR provided a statistically significant reduction the number of patients hospitalized vs. placebo+BR. The reductions in the number of hospitalizations and time spent in the hospital represent clinical benefit to the patients and significant savings in hospital-related costs to the healthcare system.