* Raltegravir, the first in a new class of orally administered HIV type-1 (HIV-1) integrase inhibitors, selectively inhibits the strand transfer activity of HIV-1 and its integration into human DNA, a key stage in retroviral propagation, thereby limiting viral replication and the infection of new cells. * In two randomized, double-blind (with in-house blinding), placebo-controlled, multicentre, ongoing phase III trials, the proportion of patients achieving HIV-1 RNA loads of <400 copies/mL (primary endpoint) was significantly greater in raltegravir plus optimized background therapy (OBT) recipients than in placebo plus OBT recipients (preliminary 24-week results). * The proportion of patients achieving viral loads of <50 copies/mL was significantly greater with raltegravir plus OBT than with placebo plus OBT in the two studies. * In addition, mean CD4+ cell counts (secondary endpoint) were significantly increased from baseline in patients receiving raltegravir plus OBT relative to those receiving placebo plus OBT. * Raltegravir therapy was well tolerated overall. The incidence of mild to moderate adverse events was similar in the raltegravir and placebo arms of the two randomized trials.