Sexually transmitted infections may enhance the sexual transmission of HIV-1. It is possible that loss of virological control in patients with such infections receiving antiretroviral therapy (ART) may even facilitate the transmission of drug-resistant HIV. We have recently demonstrated that in those on maximally suppressive ART this effect is reduced. We have examined the virus obtained from the blood plasma and seminal plasma of six HIV-1-infected men receiving poorly suppressive ART with acute urethritis for the presence of drug resistance-associated mutations. In four men with gonorrhoea the blood and seminal plasma HIV-1 had mutations conferring reduced susceptibility to one or more available drugs. In one of these men the viral load of drug-resistant virus in seminal plasma was 20-fold higher during gonorrhoea than following antibiotic treatment, with no change in blood plasma viral load. We conclude that in the presence of suboptimal ART, sexually transmitted infections may enhance the spread of drug-resistant HIV-1.