Highly active antiretroviral therapy (HAART) has led to a substantialdecline of morbidity and mortality in HIV-infected patients. However, thesuccess of HAART in treating HIV-infected patients may be jeopardized bythe occurrence of viral resistance to components of the antiretroviraldrug regimen, which is now becoming more prevalent in recently infectedpatients. Antiretroviral resistance testing can be used to guide theoptimal selection of antiretroviral drugs in patients with treatmentfailure. In this study we analyze the cost-effectiveness of antiretroviralresistance testing in HIV-infected patients with documented treatmentfailure. The cost-effectiveness analysis will be based on astate-transition model of disease progression. In a state-transition modelthe course of disease is described using a set of mutually exclusivehealth states and probabilities that describe the transitions that mayoccur between these health states. The simulation model will be populatedusing transition probabilities extracted from data of the Swiss HIV CohortStudy and the ZIEL study, and the costs will be based on estimates derivedfrom patient charts of enrollees.