The scope of this study was to evaluate the vulnerability of Brazilian youths to STDs/HIV& AIDS and Viral Hepatitis. Interviews were conducted with 1,208 youths aged 18 to 29 in 15 states and the Federal District. The regional and national adjusted margin of error of the research was 2.8%. The study was approved by the School of Medicine of Brasilia University with support from PAHO and the Ministry of Health. A scale with 35 questions (knowledge, attitudes and practices) was the main dependent variable. Adjusted linear regression models identified the demographic and social determinant factors that explain scale variations. The scale attained a satisfactory level of consistency (Cronbach's Alpha: 0.689). Socio-demographic factors associated to scale variations include gender, race, education and civil status. Social determinants associated to scale variations include frequency of discussions about sexuality with parents and health professionals, alcohol consumption, leisure and being part of a social movement, access to the internet, interest in learning and having the father and/or teacher as a personal reference. Brazilian youths are vulnerable to the transmission of STDs/HIV& AIDS. Public policies are needed to promote the engagement of parents and teachers in issues related to sexuality.