Raltegravir (Isentress), an integrase inhibitor, inhibits the insertion of HIV-1 complementary DNA into the host genome. It is indicated in combination with other antiretroviral therapy (ART) agents for the treatment of HIV-1 infection in treatment-experienced adult patients who have evidence of viral replication and HIV-1 strains resistant to multiple ART agents. It is the first of a new class of ART agents to be approved that, as a result of a different mechanism of action to other ART agents, has good activity against multidrug-resistant HIV-1 strains. In clinical trials in treatment-experienced patients with HIV-1 infection and evidence of viral replication, the addition of oral raltegravir to an optimized background therapy (OBT) regimen improved virological and immunological responses at 16 and 48 weeks to a greater extent than placebo plus OBT. Raltegravir therapy was generally well tolerated, with a similar incidence of mild to moderate adverse events in the treatment and placebo arms. The introduction of integrase inhibitors extends the options available for managing treatment-experienced patients with multiple-drug-resistant HIV-1 infection. Results to date suggest that the combination of raltegravir and OBT will be a valuable treatment option for this difficult-to-treat patient group.