According to the Centers for Disease Control and Prevention (CDC), in California, injection drug use is the leading cause of hepatitis C virus (HCV) transmission and the second leading cause of human immunodeficiency virus (HIV) transmission. There is overlap between behaviors for HIV and HCV transmission, such as sharing syringes, having multiple sexual partners, and having unprotected sex. Similar to injection drug use, homelessness is a major social problem, with public health implications. According to the National Coalition for the homeless (NCH), the homeless population is disproportionately affected by viral infections: in 2009, it was found that nearly 3.4% of homeless persons were HIV-positive. Between March 2009 and June 2010, the cross-sectional Study to Assess Hepatitis C Risk under the University of California, San Diego recruited 566 young adult injection drug users in San Diego, to identify correlates of HCV infection. The study also assessed homelessness, which permitted an analysis of the influence of homelessness on injection drug use. This study assessed the differences in injecting practices and sexual behavior in injection drug users who are homeless, compared to those that are not. Homelessness in the six months prior to the survey was examined as the dependent variable. Independent variables included behaviors related to acquiring blood-borne viral infections such as: receptive syringe sharing, receptive drug paraphernalia sharing, number of sexual partners, unprotected vaginal and anal sex with non-steady partners, and exchanging sex for drugs, money, or other necessities. Overall, homelessness was found to be significantly associated with non-regular sources of income, sharing syringes, and exchanging sex. Findings from this study suggest that recently homeless young adult injection drug users are at even greater risk for blood-borne viral infections than non-homeless young adult injection drug users. Further studies are needed to determine whether unstable housing is a cause or a consequence of these risk behaviors in order to inform HIV and HCV prevention and control interventions among high-risk subgroups of injection drug users.