Five serial cross-sectional surveys were done at eight gay bathhouses in Taiwan to investigate the trends of HIV and sexually transmitted infections (STIs) and estimated HIV incidence between 2004 and 2008. Bathhouse attendees completed a questionnaire and tests for HIV, syphilis, hepatitis C virus, and amoebiasis. Twenty-nine (38.6%) were identified as having recent HIV-1 infections. There was a significant increase in HIV incidence, from 7.8% in 2004 to 15% in 2007 (χ2 = 17.59, P-trend <0.001). Recreational drug use is the primary risk behavior. Comprehensive screening programs in gay bathhouses for early detection of HIV and STIs are important.