The Canadian "Think Again" social marketing HIV/AIDS prevention campaign, adapted from an American effort, encourages gay men to rethink their assumptions about their partners' HIV statuses and the risks of unsafe sex with them. To improve future efforts, existing HIV/AIDS prevention initiatives require critical reflection. While a formal evaluation of this campaign has been carried out elsewhere, here we use the campaign as a social marketing case study to illustrate its strengths and weaknesses, as a learning tool for other campaigns. After describing the campaign and its key results, we assess how it utilized central tenets of the social marketing process, such as formative research and the marketing mix. We then speak to the importance of theoretical influence in campaign design and the need to account for social-contextual factors in safer sex decision making. We conclude with a summary of the lessons learned from the assessment of this campaign.