This paper describes a study among HIV-negative gay men in London to examine willingness to volunteer for an HIV vaccine trial. HIV-negative gay men (n=506) were surveyed in central London gyms in February-March 2002. Information was collected on willingness to volunteer for an HIV vaccine trial, attitudes toward HIV vaccines and sexual risk behaviour. Men reporting unprotected anal intercourse (UAI) in the previous three months with a man of unknown or discordant HIV status were classified as being at high risk of exposure to HIV (n=94, 18.6%). The remainder, who reported no UAI or UAI only with another HIV-negative man, were considered to be at low risk (n=412, 81.4%). Just under a quarter of the HIV-negative men in the study (23.4%) said they were either quite likely or very likely to volunteer for an HIV vaccine trial. High-risk men were more likely to say they would volunteer for a trial than low-risk men (37.2% versus 20.2%, p<0.001). Of the 506 HIV-negative men surveyed, eight (1.6%) were both high risk and very likely to volunteer for an HIV vaccine trial, while a further 27 (5.3%) were both high risk and quite likely to volunteer. Based on these figures, we estimated that to recruit 1,000 high-risk HIV-negative men into a vaccine trial between 15,000 and 62,000 HIV-negative men would need to be approached in the community. Compared with those at low risk, a greater proportion of high-risk men said that if they were in an HIV vaccine trial they would be more likely to have unprotected sex (23.4% versus 7.8%, p<0.001); that an effective vaccine will make safe sex less important (45.7% versus 31.3%, p=0.01); and that they would participate in an HIV vaccine trial even if they thought the vaccine might not work (46.8% versus 29.9%, p<0.01). This study suggests that, in London, to recruit high-risk HIV-negative gay men for an HIV vaccine trial many thousands of gay men may need to be approached in the community. Some HIV-negative men said that they would be more likely to have unprotected sex if they took part in a trial.