The disparity between people's knowledge about HIV/AIDS and the extent to which they take measures to protect themselves is one of the most vexing issues for public health workers and social science analysts. This paper aims to explain some of this discrepancy, using survey and ethnographic data collected among young rural-urban migrants in Aba and Kano, two cities in Nigeria. The paper argues that many young Nigerian migrants do not perceive significant personal risk because they construct the risk of AIDS in ethical and moral terms, projecting immorality and danger onto imaginary others. To understand the way young Nigerians interpret risk, the paper focuses on four related issues: (1) the organization and meaning of sexual relationships; (2) the intersection of gender and ideas about reproduction; (3) the perception of AIDS as a disease without hope; and (4) the importance of religion in young people's framing of moralities and ethical choices about sexuality and HIV/AIDS.