Although syphilis is a serious disease with long-term consequences if untreated, this increase is particularly worrisome because syphilis is known to facilitate the transmission of HIV (Fleming & Wasserheit, 1999). In Los Angeles, approximately 60% of MSM diagnosed with early syphilis in 2005 were also HIV positive (STDP, Los Angeles County Department of Public Health, 2006). Furthermore, a recent study suggests that acquisition of syphilis and HIV may be occurring concurrently among MSM (Taylor et al., 2005). As part of its public health response to this syphilis outbreak, the Los Angeles County Department of Public Health STDP partnered with AIDS Healthcare Foundation (AHF) and five other community-based organizations (CBOs) in 2001 to support a social marketing campaign. This article outlines the development, implementation, and evaluation of Stop the Sores, a social marketing campaign designed to increase syphilis testing, knowledge, and awareness among MSM in Los Angeles County. Special attention will be given to the role of social marketing principles in the success of the campaign. The evaluation was conducted to assess the impact of the campaign, if any, on syphilis testing behavior among MSM. Specifically, MSM who were exposed to the campaign should be more likely to have had a syphilis test. Los Angeles County has experienced a rapid increase in early syphilis among men who have sex with men (MSM) in recent years, with the number of cases rising from 126 in 2000 to 809 in 2005. As part of the public health response to this outbreak, a sustained social marketing campaign was launched in 2002, with the objectives of increasing syphilis testing, knowledge, and awareness among MSM in Los Angeles. This campaign, as implemented, exemplified key principles of social marketing, including market research, audience segmentation, and branding. A cross-sectional study conducted in 2004 to evaluate the campaign found that those MSM who were aware of the campaign were nearly twice as likely to have tested for syphilis in the past 6 months as those MSM who were not aware of the campaign. Those MSM who were aware of the campaign also had more syphilis awareness and knowledge in key areas.