To examine differences in knowledge, attitudes, and related practices among adopters and nonadopters of the human papillomavirus (HPV) vaccine, the researchers conducted 83 in-depth interviews with 18- to 26-year-old women. The study identified knowledge-attitude-practice gaps in the context of the HPV vaccine to explain why diffusion of a preventive innovation (such as the HPV vaccine) requires targeted risk communication strategies in order to increase demand. Salient findings included similarities between vaccinated and unvaccinated women's lack of knowledge and uncertainties about HPV and cervical cancer. Vaccinated women who had no knowledge of HPV or no-risk/low-risk perceptions of HPV reported receiving vaccination, indicating HPV risk protection behavior could precede knowledge acquisition for vaccinated women. These vaccinated women identified an interpersonal network supportive of vaccination and reported supportive social influences. Among unvaccinated women, unsupportive vaccination attitudes included low perceived personal risk of HPV. In contrast, unvaccinated women often cited erroneous beliefs that HPV could be avoided by abstinence, monogamy, and knowledge of their partners' sexual history as reasons that the vaccine was not personally relevant. Unvaccinated women cited interpersonal influences that activated short- and long-term vaccination safety and efficacy concerns. Different levels of fear regarding the HPV vaccine may underlie (a) attitudinal differences between vaccinated and unvaccinated women in perceived vaccination value and (b) attitude-practice gaps.