The study purpose was to evaluate the effectiveness of a structural intervention in reducing unprotected sexual behaviours, increasing condom accessibility and their relationship to the prevalence of HIV infection and sexually transmitted infections (STIs) in gay bathhouse attendees. A quasi-experimental study with a non-equivalent control group for comparison was conducted at nine gay bathhouses in Taiwan from 2004 to 2006. A structural intervention designed to increase condom distribution and accessibility inside the bathhouses was implemented at one bathhouse in each city. Bathhouse attendees were invited to complete a questionnaire and to be screened for HIV infection and STIs. Of 632 study participants, 270 were surveyed six months after the intervention was conducted. At the time of follow-up after controlling for baseline data, patrons attending bathhouses for which intervention was performed were more likely to report accessing condoms inside bathhouses than those attending control bathhouses (92.6% versus 81.3%, P = 0.016), and condoms were more likely to be available at the reception desk of the bathhouse entrance (87.5% in intervention bathhouses versus 69.4% in control, P = 0.047). In a multivariate analysis adjusted for age, access to condoms inside bathhouses (adjusted odds ratio (AOR): 4.35; 95% confidence interval (CI): 1.48-12.78) and attendance at bathhouses with intervention (AOR: 2.07; 95% CI: 1.07-3.97) were independently associated with consistent condom use during anal intercourse. There were no significant differences in the prevalences of HIV infection and STIs at the six-month follow-up. Bathhouses with structural intervention were associated with consistent condom use during anal intercourse among bathhouse patrons. Our findings highlight the importance of extension of condom distribution inside the bathhouses to increase condom use among bathhouse patrons.