Human immunodeficiency virus (HIV) prevention delivered in gay venues in US cities has been found to be effective in reducing HIV transmission in the 1990s but effects might not be generalizable to different times and settings. Doubts have been raised about: outreach's ability to address skills and explore personal behaviour; big-city commercial gay venues being appropriate sites for outreach because of gossip and social surveillance; and acceptability of outreach by professionals rather than 'popular opinion formers'. We evaluated coverage, feasibility, acceptability and perceived impact of venue-based HIV prevention outreach by professionals in London, employing observation, surveys and interviews with venue-users, and focus groups/semi-structured interviews with workers. We found high coverage especially among target groups. Addressing negotiation skills and personal behaviour was feasible but required worker motivation and skill. Social surveillance rarely impeded work. Gay men generally found outreach acceptable and useful, and professionals were not regarded negatively. Impact on knowledge was commonly reported; impacts on negotiation skills and reflection on personal behaviour were more common among men experiencing longer contacts. In conclusion, professional HIV prevention outreach in gay venues in large cities is a feasible and acceptable intervention with significant potential impacts. Workers need to be well briefed and trained to maximize impact.