Low-income, urban, African-American and Hispanic youth have been identified as a group that may be at risk for the spread of human immunodeficiency virus (HIV). This article evaluates general knowledge of acquired immunodeficiency syndrome (AIDS), knowledge of routes of sexual transmission of HIV, risk behaviors related to sex, and perceived susceptibility to AIDS of urban low-income youth in Detroit. We drew data from a household probability sample of 1,435 of these Detroit youth. The data indicate that, with a few exceptions, general knowledge of AIDS and routes of sexual HIV transmission was good; there were small ethnic and gender differences in knowledge. However, we found substantial ethnic and gender differences in risk behaviors. Young African-American men reported the earliest initiation of sexual activity and the most partners. Young Hispanic women reported the latest initiation of sexual activity and the fewest partners. A substantial minority of the youth were concerned about becoming infected with HIV, and these concerns were related to risk behavior. We demonstrate from these data a need for interventions in this population to correct misconceptions and to promote use of condoms and other safer sexual behaviors.