Study Objective: To assess knowledge and attitudes of young uninsured women toward human papillomavirus (HPV) vaccination and clinical trials. Design: Cross-sectional study. Setting: Clinic-based sample in Brazil. Participants: A consecutive sample of 204 women aged 16 to 23 years, attending a public outpatient gynecological clinic. Interventions: A questionnaire administered by in-person interview. Main Outcome Measures: Data on knowledge and attitudes towards HPV vaccination. Results: Overall, 72% of the respondents would enroll in a HPV vaccine trial, despite the fact that 69% of women were ignorant of what HPV may cause, and only 10% acknowledged that HPV might lead to cervical cancer. The need of a placebo arm (31%) and three vaccinations injections (26%) were the trial design characteristics most cited for deterring participation. Factors promoting participation were ‘‘careful/detailed consultations by the same physician’’ (92%), ‘‘access to more information on women’s health’’ (84%), and ‘‘office visits on time’’ (79%); whereas ‘‘clinic too far from home’’ (36%), ‘‘fear of adverse events’’ (29%), and ‘‘gynecologic examination discomfort’’ (25%) were the most commonly reported reasons for not enrolling in a trial. Being sexually active, more than three lifetime sexual partners and perception of high risk for cervical cancer were predictors of participation in a HPV vaccine trial. Conclusions: Knowledge of HPV infection and cervical cancer is low in this urban, young population. Thus, when planning HPV vaccine trials, it is important to consider implementing educational programs to provide knowledge of the benefits of a preventive vaccine and information on the etiology of and risk factors for cervical cancer.