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.