Mobile unit (MU) HIV testing is an alternative method of providing healthcare access. We compared demographic and behavioural characteristics, HIV testing history and HIV prevalence between participants seeking testing at a MU vs. fixed clinic (FC) in Lima, Peru. Our analysis included men and transgender women (TW) in Lima aged ≥ 18 years old seeking HIV testing at their first visit to a community-based MU or FC from October 2007 to November 2009. HIV testing history, HIV serostatus and behavioural characteristics were analysed. A large percentage of MU attendees self-identified as transgender (13%) or heterosexual (41%). MU attendees were more likely to engage in transactional sex (24% MU vs. 10% FC, p < 0.001), use alcohol/drugs during their last sexual encounter (24% MU vs. 20% FC, p < 0.01) and/or be a first-time HIV tester (48% MU vs. 41% FC, p < 0.001). MU HIV prevalence was 9% overall and 5% among first-time testers (49% in TW and 11% in men who have sex with men [MSM] first-time testers). MU testing reached large numbers of at-risk (MSM/TW) populations engaged in unsafe sexual behaviours, making MU outreach a worthy complement to FC testing. Investigation into whether MU attendees would otherwise access HIV testing is warranted to determine the impact of MU testing. © The Author(s) 2013 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav.