Single witness: HIV-1 drug resistance assays have been shown to be of value for guiding antiretroviral therapy (ART) decisions. Different tissues or body fluids in which HIV-1 can reside may contain viruses with distinct characteristics. HIV-1 variants with genotypic resistance markers are present in the male genital tract and evolve over time on incompletely suppressive ART. The magnitude of decline in the semen HIV-RNA level in with therapy is usually similar to the effect of treatment on the blood viral burden. Not all men on ART have complete suppression of HIV-1 replication in genital tract; thus, they may shed resistant HIV-1 strains. Failure of treatment to suppress HIV-RNA levels in the blood is common, resulting in the selection of resistant HIV-1 variants. Sexual transmission of resistant variants may have a negative impact on treatment outcome in newly infected individuals and on the spread of the diseases within a population. The use of rapid assessment of HIV resistance in plasma and semen may be useful in some situations, e.g. HIV transmitted infection by sexual contact from a viraemic patient. The recognition of resistant HIV isolates in the blood and semen of patients receiving ART leads to the question of the potential role of resistance testing. In many but not all infectious diseases, the choice of therapy is guided by sensitivity testing. Recent data suggest that testing the drug sensitivity of viral isolates from the blood can benefit patient management and response to therapy, albeit at considerable cost. However, since drug resistance may evolve independently in blood and semen, the public health benefits of such testing are unproven.